In Michigan, anything under 30" is considered a "pistol" under the law.
If your gun is federally a rifle or shotgun, but is under 30", it must be registered. This is known as a "Michigan Pistol." You may put a VFG on such gun without any ramifications.
If your gun is federally a handgun, you may still put a VFG on it - however, you must apply for and pay for an AOW tax stamp first (These cost $200 to "create" an AOW, and $5 to transfer it to someone).
To explain further, if you have an AK or SCAR that is operable and under 30" when stock is folded, you must register it as a pistol. If it's overall length is under 26" once modified, it's illegal in MI.