If I have these license lines (difference being Vendor string) on my server
INCREMENT F1 Google 1.0 31-dec-19 1 VENDOR_STRING=Oracle ...
INCREMENT F1 Google 1.0 31-dec-19 1 VENDOR_STRING=Microsoft ....
If I send a Preview capability request, will I get these 2 licenses back or just one F1? In order words, will I get F1 back 2 times with one count for each or will I get just 1 F1 with a count of 2?
You can either preview the licenses on the server with a specified feature and quantity or you can preview all licensed features on the server. If you send a preview request for F1 Qty 1 you will receive one of the licenses back. If you send a preview request for F1 Qty 2 you will receive both licenses back (F1 Qty 1 twice with all feature attributes). If you preview all features you will also get both licenses back.
As an added note, if you wish to differentiate between the two F1 licenses when sending capability requests you can use feature selectors to filter which license is returned by the server. Further details are provided in the User Guides.
I will use the preview all features in that case because I wouldn't know the values assigned to the vendor string until I query the server (as its dynamic data).
On the feature selectors, yes that's what I will be using to get the required licenses after I have identified what is available.
