I am interested to explore what a Yaesu VX-8DR radio can do and would like to add an amplifier to it. One of the AMPs that seems to work with a 5W input is KPA100, I am still doubtful how the Yaesu can key into the Amp. Of, course I plan to get a J-pole antenna to work with the radio + amp. Is this a good Amp for the Yaesu? Any advice on this idea of adding an Amp for a handleld?
and welcome to ham.stackexchange.com! Sorry, but the Elecraft KPA100 won't work at all with the Yaesu VX-8DR. The VX-8DR is a handheld that runs on the 6m/2m/1.25m/70cm bands; the KPA100 works on the ham bands between 80m–10m. What's more, the KPA100 is really meant to be part of the Elecraft K2, and isn't meant to be a stand-alone amplifier.
You didn't say what your intentions are, but allow me to guess that you're shopping for your first radio, and you want both the convenience of an HT and also more power when you're operating from home. I have an ancient Yaesu FT-470 handheld and a 30 W amplifier meant for it, which I got for cheap at a hamfest. So amplifiers meant for handhelds aren't unknown, but they are fairly rare.
This is a question-and-answer site and not a forum-style site, so we usually shy away from giving personal advice; we want answers that will also be useful to other people in the future. So instead here is some more generic advice.
You might be surprised how much better an HT will do connected to a good antenna, especially one mounted up high (like the roof of a single-story building) will do compared to the "rubber duck" antenna that comes with the HT. You might not need more power at all, especially if you're mainly interested in connecting to local repeaters.
Rather than spending a lot of effort and money finding an amplifier for an HT, most people would probably just buy another radio, like a mobile rig, with more power to supplement the HT. (People use "mobile" rigs both in vehicles and at home.)
Your local ham club could probably give you lots of advice, and might also be a good source of much less expensive used radios.