Matt,
I'm really not that familiar with Mikunis, although woosh and others have had good luck with them. I don't recognize that number you mentioned, the ones I know are the VM round-slide and TM/TMX flat-slide units. A VM28 looks like it's 5mm shorter than the stock Keihin but otherwise should fit; the VM30 will definitely take some work if you want good full-throttle operation. Mounting a bigger carb to the MR175 can be a real challenge if you want to do it as Honda would (my preference). The intake manifold is a 29mm bottleneck, the cylinder intake flange is a problem when trying to mount other manifolds (CR125M / TM250 / Mikuni / etc.), the air boot will be too small, throttle cable issues, etc. The stock carb can be bored 1mm and the manifold enlarged; Honda did this on the '74 CR125M's optional carb, and it made a real difference on my (modified) MR motor at nearly all throttle positions, not just WOT.
I'd suggest keeping the small-mouth design of the stock carb if possible, which really makes a difference in low end and throttle response, especially when combined with a longer air boot. The computer said so, I didn't believe it, I was wrong.
Can you point me to a web photo of your carb?
Ray