Egbert,
I have a job (with my Erreka colleagues) every year during the Sanfermines, so I can tell you that while it's very exciting, exhilarating, to be there, it will also be expensive for you and your wife to come down from San Sebastián to the fiesta just for one night with no support services.

If you think you'll never have another chance, then consider it and the costs. Since your wife is 4 months pregnant (I see from your other post), you would want to make this experience as easy, comfortable and enjoyable as possible for her in a very chaotic environment.

I know how to make the experience both loads of fun for singles, couples and families and also relatively comfortable (in a very nice and even quiet hotel) and always enjoyable, but that's my job.

So you have
the hotel costs (triple the normal rate)
the cost for garage parking for your car,
which you would NEVER want to leave parked on the street to be either towed or broken into
(unless you come down on the bus),
the cost for the comfortable balcony space for 2,
because you will see nothing if you attempt to view the run from behind the second barricade, and even there you should be out there by 6:15 am to grab a space then *try* to hold it. But you will most likely lose it to someone more aggressive.

Those are the cost considerations. Now you need to decide whether it's worth it to you and your wife.

As for your last 2 nights, I'd choose Tarragona for the easy access to the airport rather than Vilanova I La Geltru. If you wanted a beach town just to swim, sunbathe, relax, I'd choose Sitges for prettier architecture and more character than Vilanova, but that's just me.

Hope this helps you to decide.