This isn't involved with any other timeline of mine. Rated T because, while nothing happens, there's a lot of interesting implications here. (If you feel I've misrated this, please let me know.)
Vanessa walked into the quiet bar where Isabella had asked to meet her. Isabella was sitting at the bar alone, while the bartender cleaned glasses at the other end. She was dressed sharply, as usual - indigo blouse, long black skirt, black heels, the large sapphire pendant Phineas had made for her with one of his inventions, her long black hair in a braid draped over her shoulder. Vanessa looked down at her own tight black outfit and felt underdressed. Isabella was always confident, but since she's been dating Phineas, she's just so incredibly self-assured, she thought.
She walked over and took the stool next to Isabella. The bartender came over, gave her the beer she requested, and retreated.
She turned to Isabella. "So what's up?"
Isabella looked her up and down. "Since you're dating Ferb now, I figured we should meet up and get to know one another better. And...let you know what you're getting into early enough for you to get out if that's what you want. Give you fair warning."
"Fair warning of what?"
Isabella swirled her drink in its glass, then set it on the bar. "The boys have been together almost all their lives. And so, they share everything." She looked Vanessa in the eyes and smiled. "Everything."
Vanessa's eyes widened. "Oh?"
"And now that we've been a trio so long, they share everything with me, too." Isabella's smile at the older woman became almost predatory. "I've been waiting for this for some time." With one finger Isabella hooked the collar of Vanessa's blouse and gently pulled her head forward, leaning in to kiss her.
I think I'd label this pairing 'Vanbella', since that sounds better than 'Isanessa'. Although I think the implication here is more 'Phinerbellanessa' than anything else.
We'll get back to the Fletcher-Flynn Foundation timeline tomorrow.