I hope every completion done inside minibuffer is repeatable by vertico-repeat, including commands invoked by M-x. Vertico, however, didn't include execute-extended-command as a repeatable session by default. You can be seen it by examining vertico-repeat-filter variable. So I have to customize it like this in Doom Emacs:
(after! vertico-repeat
(setq vertico-repeat-filter (remove 'execute-extended-command vertico-repeat-filter)))
The question is execute-extended-command seems to be a reasonable default completing session, why does vertico excludes it?