Don't doubt you'll find positive studies -- it was approved by the FDA. But the experience of this drug is it's been around a very long time and until recently wasn't used much anymore because it just didn't work for enough people. Someone's spending money on this drug, as it's been trying to find something to treat for a very long time, and has reappeared as an adjunct therapy to improve antidepressant performance. But it obviously does work for some people or it would never have been approved in the first place.
http://www.ncbi.nlm.nih.gov/pubmed/2649317 (comparable to diazepam for treating anxiety in a 367 female patient double blind study). I found it worked very well for my own anxiety condition, but I dislike taking meds on general principles so aggressively sought ways of discontinuing it.
Buspirone's now generic, so there's unlikely to be any funding forthcoming for newer studies.
It's most likely the amitriptyline, as buspar seldom works for anyone -- it hasn't done well in clinical trials, and its main use now is as an adjunct for an antidepressant. Not that it never works, but it's just probably much more likely it's side effects from starting the other med.
I was on Buspar (buspirone) for a year, it takes several weeks to experiment and ramp up to a minimal yet effective working dose (for me, that was 7.5mg twice a day, 5 was too little and 10 was too generous). I never experienced vivid dreams/nightmares with it. It was a challenge after the year to wean myself off it, and I had to do that very, very slowly over some 7-8 weeks.
A quick google does suggest amitriptyline can often lead to vivid dreams and nightmares for some people. I've never been on that med.