I've never noticed the P in any literature that I've read, but if I had to guess it stands for PCIe.
There is no difference between any of the blades you pull out of a M1000e enclosure or VRTX. The firmware, BIOS, management engine etc is all the same.
The only difference is that the blades that ship with the VRTX or are sold as "for PowerEdge VRTX" have PCIe mezzanine cards instead of add-on mezz cards.
Just curious, what is your budget for buying a VRTX and where do you plan on buying it? If eBay, be careful about who you buy from. I would honestly not buy any VRTX enclosures from sellers who do not ship it via freight. It is too heavy and too big to send via regular ground shipping and I can almost guarantee it would arrive damaged if it were. It's designed to be shipped strapped to a pallet.