Gabrielli wrote:

PS: We've nearly finished gutting and remodeling our bathroom, so we've been
able to figure out or research most things, but the web has precious little
info about replacement-less replacement window installing. Go figure.

That is because replacement windows aren't designed to be installed
that way. If you have the time order the correct sized new
installation window. Pulling off a little bit of siding will probably
be much easier than what you are dealing with now.

If you can't wait then you need to weatherproof the window on the
outside then worry about the inside. It sounds like you are mounting
this window high so water from the shower shouldn't be a major concern.
I would put some metal flashing on the outside so that water can't get
in then try and make it look right. Replacement windows require
moulding on both sides to cover up the gaps.

When you are ready to screw it into the studs make sure you use shims
between the window and the studs so the frame doesn't warp when you
tighten it up. Also spray in the expanding foam designed for windows &
doors, again so the frame doesn't warp (gap sealing foam can easily
compress a vinyl window frame.