I have a signalhound USB spectrum analyzer. It performs acceptably for my needs, which is measuring the modulation spectrum of a laser. It has a tracking generator option, but I am not sure if it can do VNA mode or only SNA. I haven't really done any tests on the noise floor or timebase stability, my signals tend to be quite bright and narrow, and they drift more than the SA would anyway: something like a 3 GHz carrier with 2-3 sidebands at several MHz. The biggest problem I have is that the software is a bit clunky. Most importantly, it is a bit slow -- both the spectrum analyzer and the software. The spectrum analyzer only has 240 kHz of realtime bandwidth except in the low resolution 5 MHz RBW mode. Also the UI controls don't respond except at the end of a sweep. If you are at a high resolution narrow span and the signal you are watching drifts off the screen, when you bump up the span you can easily get to 30 second sweeps and it doesn't appear to let you cancel or increase the RBW until the 30 seconds are up. A 4.4 GHz SA for $1000 + $500 for the tracking generator it is not a bad deal, but on the other hand I am used to a bit more polish in a $1500 product.
If all you need is SWR you can do that with a coupler / circulator and a power meter.