Dealing with Vibration on 2d Floor of Home

You want to brace the rack against a wall. Strongest one you can (external, preferably). You could also try wall mount (assuming TT isn’t too large) but I like the wall bracing because it’s easy and combines strength of both wall & floor. In that configuration, your turntable is no longer "cantilevered" into space in any way. Using foam or ribber shims to interface, just push the rack firmly against the wall. 

That will clean up the bulk of shockwaves (footfall) and subsonic energy. If you still have leftover feedback in the audible range, a Townshend podium or pods is the best reasonable solution to mop that up. For bass frequencies (the toughest to handle due to amplitude) it’s much more effective than constrained layer damping or polymer or bearing feet, imo. 

A freestanding tall narrow audiophile tower rack on a suspended floor is about the WORST way to run a turntable. Sadly, this is how many are run. It needs bracing. 

I have the same issue, tried an IsoAcoustic platform and it didn’t do much.

I then switched to a Townshend Audio Seismic platform and like @bdp24 I can highly recommend it. Their platforms are optimized based on mass, so you tell them how much your tt weighs and they tune the platform’s isolation to your particular tt.

A wall mounted platform is another potential solution, as others have mentioned but 1) it may not work for your setup in terms of reaching the turntable conveniently if mounted on a wall, 2) not everyone likes to have a turntable attached to the wall from a visual perspective rather than on a console or rack, and 3) how effective a wall mount is actually depends on your wall.

Just like floors, all walls are not born the same. 

I dealt with a similar issue years ago.  Mounting the turntable on a shelf on the wall solved the issue.  Best of luck.  

This is exactly why wall shelves were invented for turntables... fifty or more years ago. 

Check out the Townshend Audio Seismic line of products. I use the Podiums for my speakers, and it is an incredible product. The podiums are vibrating like crazy, and my speakers are perfectly still. Chances are they have something for a turntable.