:help vimtex-compiler states the following:

It does not support continuous
compilation like |vimtex-compiler-latexmk|, so the only relevant commands are
|:VimtexCompile| to start (single shot) compilation, and
|:VimtexCompileOutput| to see the compilation output. 

But that's not quite true (anymore).
The tectonic V2 CLI supports continuous compilation:

Starting continuous compilation
tectonic -X watch

Starting CC and open file (like latexmk -pvc)
tectonic -X watch --exec "build --open"

I would like for vimtex to have support for this.

Here is a reference page from tectonic for this: V2CLI Watch