I'm going to beg to differ and say you won't be happy with a VNA for SA work.
It's possible but it sucks.
Hi Hendorog. I would be eager to hear where you think the VNA would be lacking. Noise floor issues, missing features, user interface unsuitable or something else?
A VNA lacks the hardware and design required to make a good SA, the issues are as described already by rf-messkopf's post. I would add potentially poor phase noise and maybe spurs to that list and also the coupler/bridges reduce dynamic range further unless the unit has direct receiver access.
Another potential consideration, is that old VNAs, such as my 8753, used a pulse generator to downconvert the external signal into the receivers. Which means you would need to provide a front end set of mixers as well and drive it from the source. The mixers would need to produce the IF at the same freq as the source.
There is a receiver mode in the 8753, but it is a long time since I've looked at that. I thought it maybe could be a poor man's SA too, but after some playing around I remember deciding that it wasn't worth further effort due to the coherent source issue.
The software is a problem. Someone might have tried it before on your device, but it's a far cry from a purpose built SA.
As tautech points out, combined VNA/SA devices exist, work well, and I have one. But using a pure VNA as an SA would be painful.
There is an old thread here which asked the same question, and some dual SA/VNA devices were mentioned in that.
A reverse example, the Signalhound SA range can be used as a VNA, but only for a single frequency at a time. A new calibration must be done after each frequency change.
I recall that some of the very low cost vnas like librevna and nanovna do have an SA mode which work with limitations. If so then a nanovna in particular doesn't cost much money, so you could try see for yourself if it will work for what you need.