There has to be a floor.
Yes,Vinyl should go over the underlayment or the vinyl doesn't wear
well. I don't think your contractor is trying to jack up his bill.
Having done that sort of work myself I would refuse to put down vinyl
without underlayment bcz I'd be uncomfortable doing a job in a way
that would ultimately hurt my reputation, if not my conscience. It'd
just be a bad job and it would show and you'd resent it and you'd
tell people who did it etcetc.
But look at it this way - It is only a small studio and how much
floor is actually exposed once you allow for cabinets etc? Maybe it
isn't such a big job? And how much traffic will it get? Kids? Dogs?
Cocktail parties? Disco dancing?
My metalwork studio is just plywood floor - unpainted. At some point
(when I get around to it) I'll paint the floor with a uniform color
as I have (sometimes) done in other studios before because it is a
good way to make a clean appearance & makes it easier to see dropped
objects. Since I don't look at the floor all that much (I bet you
don't either) the exact shade of color is not generally a critical
matter. Painting a small floor is a good job to do when you just need
a break. Paint stores and other such places often have huge
quantities and selections of mis-tinted paints (their screw-ups)
which can be had for very little money. When the floor gets too
messed up just wash it and paint again. People don't expect a
workshop to have a floor like a ballroom - though I have seen some
like that - daunting!
My wife's painting studio has a nice wooden floor which we wish to
preserve in original condition so I covered it with masonite - also
known as "hardboard" here in Canada. Double-faced tape under the
seams keeps it stuck to the floor and keeps paint and such from
getting through the seams to damage the floor underneath and is
removeable eventually should we wish to restore that particular room
to its original "heritage" status. The main thing - masonite is very
smooth - smoother than plywood- and lots cheaper than vinyl - also
easily paintable. Relatively resistant to water and other such
disturbances IF you use the "oil-tempered" variety and if you paint
it and (finally) if you don't leave wet things or puddles on the
floor for extended periods. You can wash it, mop it etc.
I suppose you could also look for roll-ends or rejects or oddlots of
vinyl and just tack it down around the dges and resign yrself to
replacing it sometime -
Marty in Victoria