I'm going to be replacing my windows soon. I plan to use minimal
expanding foam to fill the gaps.

The Great Stuff for windows (blue can, "Will not bow frames") says
uncured foam can be removed with acetone. Can I assume that the
acetone will not harm the vinyl?

I've been practicing trying to neatly fill a 1/8" gap with the
supplied straw and it's pretty tough, even after pinching the straw so
it will fit into the gap.

Do I need to put foam in a 1/8" (or smaller) gap?

(This is the side gap I expect since I'm told my windows will be 1/4"
narrower than the rough opening.)

Thanks!