It has been quite long time since I used ADS for the last time! I believe you can do it very easily on ADS...
First, I'd like to say that as u may know, VSWR is a measurement of loading effect on signal at transmission lines, or in other words, how big portion of the signal would propagate through the load. If that is what u are trying to measure for ur VCO, then let's go...
If u have already finished designing your VCO (behavior or circuit level), prepare a test-bench in ADS including Smith Chart drawing between the two ends between which u need to measure VSWR. (Refer to this explanation: https://cnx.org/content/m1059/latest/)
Using common Smith chart basics, it'll be very easy to calculate any parameter u want..
Also, find attached a made-by-cadence VCO design guide using Spectre RF. It may also help..