There is a thread here somewhere showing pics of the inside of a vmax starter. The pics show the brush plate and corrosion that can occur in there. I took my starter apart, cleaned all the connections between that brush plate and the starter casing. By connections, I mean where the two go together and tighten together when the casing screws are put back on and tightened. There is no physical wiring connector from that brush plate to the casing. There should have been one from the factory as that continuity is the only way to properly ground the brushes.
Anyway, I put the slightest bend in the two or three little tabs from the brush plate to the casing so that when re-assembling the starter, there was much more pressure needed to tighten the screws, thus causing a much tighter connection between the brush plate and the starter casing. Problem solved FOR ME. (Re-lubing that front starter bearing never hurts either.)
It now turns over just as easy with a hot motor as it does first thing in the morning with a cold motor.
Tweeking your charging system, or a new battery will give more cranking amps while starting and buy you some time with that starter of course, and did with my problem, but in the end, I incurred the same problem with a brand new battery installed also and had to tear into the starter.
Good luck.