I don’t believe that is a problem (as long as you have time) because many mentors are working with an FRC team and an FLL team (or many FLL teams!)… However the mentor and students of the Vex team must be “new to FIRST”…
I work with multiple teams all the time. We mentor at least one rookie each year and I will help with any team that asks at any competition. Of course there is no rule, at least I don’t think there is. Otherwise I’m in BIG trouble.
The “new to FIRST” rule is in place to encourage new participation in FIRST, however there are no FVC “mentor police”. If you are doing a good thing for kids who weren’t in FIRST before, then go do it! See if you can’t drag a new inspiring adult into the mix too!!
Speaking as someone who mentored two teams last year (and I mentored both teams “full time”), I wouldn’t recommend doing it. It takes a lot of time and a lot of work. I ended up lucky in the fact that only one of the teams had a large crunch time before the ship date (oddly enough, it was the non-rookie team that did). It also helped me personally that I had two different roles on the different teams (on the rookie team, more general engineering; on the non-rookie team, programming).
All in all, it’s not something I’m likely to do again.
Members of FVC teams registered for an FVC Pilot Program event are expected to be high-school-aged young people and adult mentors who are new to FIRST
I am not sure how this affects you but it is an interesting rule FIRST has made.