Scenerio:

1000 sf of gluedown engineered hardwood with 250 sf being vinyl in the kitchen area. Vinyl has two layers with first layer originally installed in '88. Safe to glue over the vinyl with Bostiks or remove it?

How about Dri Tac? Safer?

I'd remove the vinyl. In my experience, the adhesive for vinyl can lose it's bond after many years and exposure to moisture. To glue over two layers of old vinyl on a slab would be quite risky, IMHO.

Ditto

So what do you guys do nowadays when, for example you go into a new home and they want to glue and engineered hardwood over the vinyl? From my experience urethane adhesives will pull latex ones from their bond.

Reason why is a guy from Cal called me last week and he spent all this time putting down a vinyl moisture barrier in preparation for glueing down an engineered. I told him it's not going to work with Bostiks, but what about DriTac?

An easy answer. Go with Franklin Advantage. It is made for gluing down over vinyl vapor barriers. It used to be called 811, then 811 plus, then something else but it is Franklin's top urethane flooring adhesive and is widely recommended in lieu of Bostick's. I don't know about Dri-Tack.