VPI felt mat vs aftermarket

What do you mean by a proper VPI platter?

My VPI has the standard Classic platter with the indent for the record label area around the spindle. Only indent is much deeper than thickness of a typical paper label, but more shallow than the rubber ring VPI supplies for their clamp. So I'm not exactly sure what it supposed to accomplish.

You’re sporting a proper platter.

Yes, with a reflex clamp the washer (rubber) is meant to raise the record just slightly above the platter surface. The threaded reflex clamp forces the record over the washer due to the sculpted profile of its underside. As it is tightened, it forces the outer perimeter of a record to contact the platter surface. This contact patch moves closer to the lead-out groove & label area as the clamp is tightened.

" I tried using VPIs stainless steel screw down clamp with rubber gasket, but I don't find it performing any better than HRX weight with warped records"

dmk_hifi - the amount of dish provided by the clamp/washer is limited by the degree/area of warp the LP has. 

I play used LP almost exclusively. I don't play LP's beyond what the clamp will fix.

A slightly thicker washer (for more dish) MIGHT help for those? 

There are some less costly  than VPI periphery ring solutions. None of Agon at the moment.