Eight minutes. Eight minutes is the average arrival time of a London ambulance, and John estimates that there might be two and a half minutes gone since Sherlock shot the bomb. He also estimates that with the likelihood of an ambulance being on the way once the explosion gets phoned in, then at the very most he has ten minutes to make up his mind.
But Sherlock doesn't have ten minutes, not at the rate that he's bleeding, even with John pinching his IVC to stop the blood flow. So there's only one thing left to do, really.
He withdraws his fingers from the bullet hole, and tears a strip of cloth off Sherlock's shirt, carefully packing it into the wound like he's done hundreds of times before back in Korea, and repeating the process with the hole through his leg. Sherlock never stirs, never even moans, and John's half hoped that the pain would bring him out of it.
Evidently not. Onto step two.
John rolls back his sleeve, and dips his tongue into Sherlock's blood. His fangs extend just as they're meant to, and he sinks them into his arm, tearing a gash when he pulls them out again. The dark blood half-oozes, half-slides, not at all like the way that it would gush if he were human. Carefully he props up Sherlock's limp head, and puts the blood to his lips. It dribbles down his chin, and even unconscious Sherlock gags on the taste, John silently willing him to drink it, which he does after a moment, taking deep gulps as if it will give him life instead of taking it. (And John really hates himself for this, but Sherlock's told him it's what he wants, and John knows that if he lasted as far as the hospital (unlikely) then he wouldn't last too much longer, so it has to be this way. But it doesn't have to come easy for the vampire. Though, he now knows, he'd do it anyway. Because Sherlock always has to exist, in some form or another. He can't just be . . . gone.)
When John senses Sherlock's drank enough blood (and so much of this is instinct), he withdraws his arm, binding it tightly with more of Sherlock's shirt. They only have a handful of minutes before the ambulance and police are bound to arrive, so John kisses his dying lover softly on the forehead, before scooping him into his arms and running.