I use stterm. That in mind, here are a few suggestions:
1) use one of the truecolor test scripts to verify that your xterm implements truecolor. This is useful not only for obvious reasons, but as a test of any intermediaries that might be in place (in my case tmux). If you can get the test scripts to output truecolor, you should be able to get vim to work.
2) Vim's recognition of terminal support is not perfect. The ":help termguicolors" entry suggests a few ":set &t..." settings. FWIW those did not work for me, but similar strings (google searched) did work. Without the correct strings, the visual results of setting termguicolors will be obviously bad (essentially no colors).