Unvested founder shares don’t impact the economics.
What do, as you note, is (x) the % of shares vested, and (y) any “single trigger” clauses that result in acceleration to 100% vested (or some other number) immediately upon an acquisition.
VCs don’t like “single trigger” M&A because acquirers usually won’t allow them, per se, and the resolution of that issue can change the economics of an acquisition.
Acquirers aren’t buying a tech co. so the founders leave the day the deal closes, usually. So the acquirer will either renegotiate a single trigger, in essence, by imposing new vesting via “holdbacks”; ask the founders to waive all or part of the single trigger; OR, allow it, but carve out some $$$ from the deal that is now subject to vesting for the founders.
A so-called “double trigger”, by contrast, VCs are usually fine with, in whole or in part. This says you get all, or a chunk, of your vesting if you are terminated without cause after an acquisition. This does not impact VC economics, and most acquirers are OK with this, though they may renegotiate parts of it.
Maybe the real point at time of VC investment is if you push to hard on 100% vesting in general, in not having vesting, both at the time of investment and on the date of acquisition .. it makes VCs feel like interests are not aligned. This spooks VCs.