Is it ever necessary to throw Pad Hopper Frogs into vegetation on a river or is that mainly a pond thing? I'm trying to expand into topwaters 

I would throw the frog in a lake, river, pond, stream... Bass will attack the frog, go for it

Bass will eat frogs anywhere, doesn't even need to be any frogs around. Get a frog with a light colored belly and walk it in open water so it looks like a baitfish, no vegetation required!

If they are hitting topwater, then they will hit a frog.

Not necessary, but if you were smart you would! 

Man get yourself a few of those frogs! you'll be amazed! 

I just caught my first topwater frog fish yesterday. Id say throw it anywhere. Very fun.