I just finished there stands the grass quest and I had veronica and rex in my party. I turned the quest in and fast traveled to sunset sarsaparilla headquarters and only rex is there?

Something about the elevator in Vault 22, or something. /vagueness

This has happened to me with Boone, just try waiting 24 hours and it should say "Veronica has returned 188 Trading Post". It worked for me.

There is a bug that when you fast travel, sometimes the AI that tell your follower where to go hasn't updated your location.

So they ran to your old location. Often you will see them zip past you.

Just wait a few hours (or a day) and they will pop up right next to you.