Buy High Plains Brass with confidence. It is very good. Another source for formed 20VT is Sleepinggiantbrass.com out of Aberdeen SD. I have done a comprehensive review of both of these on Saubier.com which is where you should go for the sub calibers.
If using .223 brass or Lapua or Nosler .221 brass go with a .234 or .235 neck. If using factory rem 221 brass the .232 neck works. Otherwise you will have to turn necks.
You will also need an action designed specifically to work with a cartridge this short unless you want failures to feed, function, eject.
