trying to rescue bland chicken soup that is already quite thick
I always equate &quot;depth&quot; with &quot;time&quot;.
The longer it simmers, the more depth it gains.
Adding anything you mentioned at the end may punch up the flavour, but I'd consider it to be 'top end' rather than 'depth'.
My first thought would be to drop it in a slow cooker &amp; see how it is in 4-6 hours - with or without your added ingredients, though definitely half the salt you are considering adding if it really is under-salted. Salt during the process seems more effective than at the end. However, you can always lift it a little more at the end more easily than taking it out again if you over-do it ;)
I'd also consider 'cheating' with a good chicken or veg stock cube.
I'd be wary of vinegar, I'd be more inclined to butter &amp;/or cream.