Let's see if I can figure this one out.
T = HP x 5252/rpm
2500 inch lbs is about 208 ft. lbs.
20 * 5252 = 105040/6000 = 17.5 ft lbs of torque
This is according to the formula at
http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm
So, to get 200 ft lbs you need a 12:1 speed reducer (208/17.5 = 11.88) and indeed a 12:1 reducer is exactly what Chuckey said.
As far as a VFD goes, you can usually get excellent torque at 30 hz using a V/Hz type VFD. You can get excellent torque from a SV VFD at 10 hz or below.
Here are a couple of charts showing the difference.
The one on the left is a V/hz VFD and the one on the right is a SV type drive.
As you can see the SV (Sensorless Vector) drive still produces 100% torque at 10hz.
VFDs will not reduce speed and increase torque like a gearbox or pulley arrangement will. They reduce speed but the torque remains the same (within reason).
A 20 hp motor at 1725 RPM yields about 45 ft. lbs.