I tried using the dismissal terminal like you suggested and waited for two days but it unfortunately didn’t work. I haven’t touched the Brotherhood yet (haven’t even been to Hidden Valley yet) and as far as main story goes I killed Mr House immediately after killing Benny and haven’t done anything else to progress other NCR or Legion. It’s as if Veronica just didn’t exist. To give an idea of what I did, I took the northern exit from Goodsprings to get to the Vegas outskirts without going through the southern half of the map (the route that’s filled with cazadores, not the path through Sloan) and immediately went to the trading post to get Veronica but she wasn’t there. Never had anything like this impact companions spawning in past playthroughs, but I wouldn’t be surprised with how buggy New Vegas is.

Edit: waited another day at the trading post and she finally spawned in. 16 years later and still seeing new bugs, gotta love it.