Languedoc enroute to Vaison la romaine in Provence

I really loved Collioure, and did not like Carcassone (maybe because I had one of the worst meals I ever had in France). Plus, it's a little out of the way on the Collioure-Vaison route. On the other hand, Sete is right on the route, and is really enjoyable.

I, also, am not a fan of Carcassonne. The streets lined with tacky souvenir shops put me right off. A second night in Collioure could be nice. There's plenty to see in Perpignan and in the nearby Pyrenean valleys. I wonder about your assumption that you'll be easily able to get a 2nd night's stay without advance reservation. You might try looking on booking.com just to see how availability is shaping up. One-night stays are time wasters and not so much fun.

I loved the little village of Gigondas, and the wine tasting there.

Well - I will be a dissenting voice and say that I didn't think I'd find much to do in Carcassonne, but we actually spent an enjoyable morning and half the afternoon exploring the fortress. We started out walking between the walls and enjoying some great views. Just missed a men's choir singing beautifully in the church (we caught the last minute - bummer). I actually liked some of the shops - yes, there were lots of tacky ones, but I did find a lovely one that sold local artisans handiwork (I have two prints just waiting to go up on my wall). And we did the audio tour of the fortress and found it very informative and worth the price.

We stayed two nights, but the 1st we didn't arrive until after supper due to a 3 hr train delay getting to Avignon from Paris (we should have arrived midafternoon). We had a car and stayed at an airbnb that was about a 10 min walk to the fortress and 10-15 min to the downtown, so we just parked the car.

There wasn't much in the downtown area that we did, but we really didn't know what to see, so we just wandered aimlessly for an hour or two before eating supper.

Would I go again? No - but like Stonehenge, I'm glad I saw it the once. I think you could just do it as a day trip if you didn't want to commit a night to it.

Carcassonne seems to be a very personal matter of taste if this forum is any indication...some people love it and others say "meh". I am in the "meh" category. It is interesting and unique but, as you would have to go out of your way to see it, I wouldn't say it is worth it. It is an imagined 1800's "rebuild" on the walls of the original city and is very touristy. It's fun if you have the time and it's right on your route but it would be a detour from your already long drive.
Collioure is a very pretty little town but very touristy as well. I would stay one night.
Some of the other places along the way that would be worth a stop are Pezenas, Montpellier, or Narbonne.
Pezenas is an old, beautiful "village circulade" with winding streets and a very regional feel. It would be about halfway between Collioure and Vaison. There's a unique winery just outside of town called Abbaye de Valmagne that is worth a stop.
Narbonnne has a 1/2 finished cathedral that is very interesting.
Montpellier is a larger place with a beautiful town square and a lively feel. I think this is one of the most overlooked towns in the south of France!