Would a Mod for BI Work With Vanilla v1.5?

Well?

Short answer: no.

Longer version: Depends. BI has a couple of unique features (Horde, swimming, Shield wall, Schiltron) that are not included in the RTW 1.5 engine. So, a mod using any of these won´t work with 1.5. With some work (see about adding units) you can, however, use the models and units from a BI mod in the 1.5 game, just without the features mentioned above.

*Looks at Unit Formations*

Haha! And you thought you could stop me!