VFD

http://www.hse.gov.uk/pubns/books/gs38.htm

http://www.newequipment.com/media/Company/32919740000ENGA_00000000332.pdf

Current; use an amp-clamp.

Voltage; A digital multimeter operates on it's own clock. If the VFD output frequency happens to fall on a harmonic of this clock the meter will never settle down, so use an analog if you can. Also, remember, output voltage will be proportional to frequency based on input voltage.

If you use a 1000:1 CT, for example, you mostly get a good reading of current. The voltage is much more difficult, the easy answer is an (older?) analogue multimeter with moving indicator rather than a DMM, otherwise, you need quite a good quality analyser to get an accurate result.

These readings are normally part of the VFD display, good quality VFD, good readings. Why do you want to measure separately?