A/N: I've actually wanted to write a !Smart Crabbe/Goyle story for a while, and while I didn't really expect this story to become that, I hope that I've made it a little easier for you to swallow the idea. As always, the Potterverse is J.K. Rowling's sandbox; I'm just playing in it.
Vincent Crabbe chased Weasley throughout the Room of Requirement, never letting him get out of sight. He knew he needed to maintain his deception until he was sure Malfoy was out of hearing range. He couldn't guarantee that Goyle would be revived in time to tell Granger of the switch, so he had to make sure the redhead kept running until he could be told the truth.
The irony of the whole mission was that the people entrusted with running it wouldn't have been thought by the rest of the Wizarding world to be capable of pulling off such a deception for so long. In fact, it would surprise most of the Wizarding world that the perpetrators were capable of properly spelling their own names. The ruse had been so ingrained into society that few had considered that the entire thing might be an act. But the Goyles and Crabbes were nothing if not meticulous in their planning and execution.
The plan had been hatched shortly before Voldemort's first rise. The Crabbes and Goyles were having their usually weekly lunch, an impressively intellectual affair, when the idea was raised of how to keep their families safe during the coming months and years. The safest course of action, they reasoned, was to make sure they could be valuable to any side in the conflict without raising the suspicions of anyone involved. But how, they wondered, could they pull off such a massive gambit? After much deliberation, Vincent Crabbe, Sr. finally had a moment of inspiration.
"If we can convince the Dark Lord that seeming less impressive than we are can be a valuable tool for him, and get the rest of the Wizarding World to believe us to be mere fools, we might be able to survive this conflict whole and untarnished," opined Crabbe, Sr. "Have you all heard of the Fidelius Charm?"
His statement was met by a round of slow nods, as more and more of those present began to realize what he had in mind.
"If we hide the fact that we are talented, we can stay out of the spotlight while still being actively involved. This should give us enough cover to emerge from the fight able to blend into the winning side, whether by force or by tact," said Gregory Goyle, Sr.
Thus, the Goyles and Crabbes bound their families to a Fidelius, with the secret of their true magical ability hidden from the rest of the world. As the years passed and the families grew, their secrecy held, such that even their close allies were unaware of their true power, with the obvious exception of Lord Voldemort, who appreciated the cunning of the plan. He had not been told of the full nature of the idea (after all, no one appreciates being schemed against), so when he was defeated by a young boy named Harry Potter, the Crabbes and Goyles were able to convince the Ministry that they couldn't be held responsible for their actions, as they weren't entirely sure what they had done. They remained in close touch with the Malfoy family, as both Crabbe and Goyle were suspicious of the influence Lucius had, and wanted to make sure he didn't ruin their carefully crafted scheme.
And so it came to pass that Vincent Crabbe, Jr. and Gregory Goyle, Jr. grew up as boyhood friends of Draco Malfoy, even becoming his official bodyguards, a position that brought a slight bit of gold to each boy. When they started Hogwarts, the Sorting Hat complemented each of them on their deception before immediately sorting them into Slytherin.
Their school years passed relatively untroubled, as every professor came to expect very little from them, so they did just enough to scrape As in their classwork while secretly studying to keep in top shape. It wouldn't do for them to lose contact with Draco, after all, and making low passing grades kept the gambit in force quite nicely.
Hearing that Voldemort had returned was troubling news for the boys, as he was the only person outside their families who had any knowledge that there was more to them than what people saw. However, it appeared the Dark Lord didn't care or was more concerned with other things, allowing the boys to stay hidden.
Following the trials and tribulations of one Harry Potter made for an interesting 7th year for the boys. At Christmas, they decided to ask their fathers, who were Secret Keepers for their respective families, for permission to bring Harry Potter's Golden Trio in on the family secret, as they could no longer be sure of who the winning side in the war would be. Though they were quite aware none of the three were at Hogwarts, Vincent Jr. believed that when the final battle was waged, the school would be the logical staging ground. The fathers conferred with the boys, and told them to expect a decision by Easter.
Easter came, and with it a scrap of parchment for each of the boys, charmed with a phrase so it wouldn't accidentally be seen by anyone who shouldn't see it. Rumors were spreading that Potter was heading towards Hogwarts, so the boys prepared themselves in case drastic measures had to be taken.
Eventually, Potter and friends did arrive at the Castle. Continuing to be the dutiful bodyguards of Malfoy, Vincent and Gregory were with him when he confronted the Trio in the Room of Requirement. A quick verbal spat was followed by an exchange of spellfire, with an emphasis on not killing anyone who might be too important. The fight quickly broke down into three separate one-on-one duels between the aisles, and Vincent decided to take his chance then and there. Briefly thinking of using Fiendfyre to cover the noise of what he was about to do, but discarding the idea after remembering that he hadn't been able to master the spell for real, he cast a Trip Jinx at Weasley, hitting him squarely. Following that with a quick Silencio, he got very close to the struggling redhead.
"Listen closely, and do not try to hex me," said Crabbe quietly, batting the other boy's wand out of his hand before he could cast anything. "The Crabbe family legacy contains magic far greater than the world assumes."
Weasley's reaction to the secret was confusion, understanding, and a slight bit of fear all rolled into one. Crabbe snuck a quick look around him to make sure no one was nearby before continuing.
"The Noble House of Goyle has magic beyond the reckoning of the Wizarding World. And now that you hold both of our family's secrets, I want you to do one more thing. Spread the word to Potter and the rest of your side not to fire on my family or Greg's, and this might not be the last time tonight we surprise you. Now, get out of here. Finite Incantatum."
Crabbe took a step back as Weasley took off running. Making no effort to chase him now that the secret was out, he instead went to look for where Greg had been stunned, and hoped that he could maintain his mask of ignorance for a few hours longer. Never again did he want to be thought of as a simpleton with a wand. With luck, tonight would be the first public display of the magical force known as Vincent Crabbe, Jr.