The idea of vesting is to prove that founders can build value before getting rewarded with their stock. When founders start, they usually need to build in some form of vesting to prevent one of them from just walking away with a windfall while the others continue to work hard to build value in the venture. Even then, however, if founders have already build some value before the formal structure is put in place, they will take their restricted stock grants with some portions immediately vested (usually 20% or so, maybe up to 33%). At Series A, the investors might insist that founders restructure their stock positions so that they have to vest at least a significant part over some period. This can vary but usually means that the founders get cut back so that only, say, one-third of their stock is vested, with the balance subject to vesting over a few years. This ensures that the investors will not get screwed and that the founders will earn out their positions as they use the investors' money to continue to build value. Finally, at the M&A stage, the purchase price is sometimes divided between a cash/stock portion that is given outright to the stockholders and another portion (usually an option grant) that needs to be earned out. The basic idea behind such a division is that x amount rewards them for the value they have built and the balance will reward them for continuing to add value in the future. Usually, the x part is by far the largest part of the consideration, with the balance (the part that needs to be earned going forward) amounting to, say, 10 or 20% of the total purchase price.
The consistent theme in all such cases is to make sure that those who have built value get non-forfeitable equity as a reward while those who need to prove themselves going forward get equity that can be forfeited.
If you have built true value, then, of $10M and you take your payment in stock that is 100% forfeitable, you set it up where you can be cheated out of all the value you have built with little or no legal recourse.
This is a HUGE red flag. I have seen founders do such deals and have begged and implored them, at the very least, to insist on 100% acceleration clauses in their employment arrangements should they be terminated without good cause. In the one case where the founders went through anyway without such protection, the company (a prominent public company) wound up terminating one of the main founders within months and all he got was a few crumbs for years worth of effort.
Check with a good M&A lawyer on this and then use your best judgment. It is ultimately your call, whatever the legal risks. But do it with open eyes and that means getting good help in assessing what those risks are.