I hope this helps.
Q. What's the difference between VPX and VPT files?
A. VPX is the file extension for tables created using Visual Pinball 10.x.x. VPT is the file extension for older Visual Pinball version (9.x.x and 8.x.x).
Q, Are VPX files created using version 10 of Visual Pinball? And VPT from the previous versions?
A. Yes and yes.
Q. Will I run into compatibility issues loading VPT files into Visual Pinball 10? Or vice-versa?
A. Yes. VPT files will open with VPX but they will more than likely error out or not play at all. They need to be converted. There is a lot of differences in how a table is created in VP9 vs VP10. VPX tables will not open at all with Visual Pinball 9.x.x and lower.
Q. Will I run into compatibility issues using PinballX?
A. Nope. PinballX and other front ends can be setup with the different versions of Visual Pinball but you need to setup the correct version for the table file you downloaded and open the table files with the correct VP version. Some tables require VP995 or 10.7.x and some of the very newest releases are for VPX8 (10.8.x currently in beta). The author of the table may have listed the version required where you downloaded the file or it could be in the script. Trial and error works too. I found that most VP9 tables work great with the latest VP9 version and most VP10 tables work great with the latest VP10 version.
Q. In downloading .vpx files, I realize pretty much everything in the past few years is in .vpx format while everything that is currently in my cabinet is in .vpt format. If I run into issues using .vpx files should I look for a compatible .vpt file?
A. There is still a lot of tables released for VP9 that have not been converted to VPX. I am sure there are a lot released in VP8 that have not been converted as well. Typically, the VPX version is going to look visually better, have better physics, and overall gameplay. I only use a VP9 tables if I really like it and there is no VPX version.