I am not sure how the VR 510 has the spindle mounted to z axis linear guides, but I would start with checking the spindle to monting plate bolts. I have had issues on the small machine we have after aggressively taking too heavy of a cut while surfacing a 4x9' walnute butcher block top. The machine did the cuts fine, but the vibration was more than I normally have. When I went to surface the spoilboard I ended up with what you described.
What I discoverd was the torque and contiued overloading of the spindle had tweeked the z axis mounting to where is was just a hair off from square tothe table in the x axis. It was a very small amount, but when you ad the length of the spindle motor and tooling the error was substantial at the table.
For us I just loosend the mounting hardware, re squared the spindle to the table and tightend it up.
Maybe this will help in looking for the issue
you can also check your spindle for runout with a dial indicator and a magnetic base. That can cause the issue too, but my money would be on number 1