Is there a way to replicate the results of a Vari ND filter like Singh Rays to blur moving water via increased exposure times in Photoshop?

In simple terms the answer is no (if I understand correctly). What you may be able to do is over expose the original exposure to get a longer exposure time, and then compensate in RAW processing - provided you don't blow out the highlights.

The other method is to take multiple exposures and then blend them together in photoshop - though in reality this never quite produces the same effect as a single long exposure.

You can stack ND filters until you reach the desired exposure level. You can also try with polarizers.
The Singh-Ray Vari-ND is not cheap but buying 3 or 4 ND filters is not going to be any cheaper. Stacking filters will also certainy degrade image quality.

I've not seen the Vari-ND filter in person and from the photos on Singh-Ray website, it looks like it's quite thick. I wonder if this could cause poblem with wide-angle lenses.

SO would you then go to a Cokin/Lee system with 6x 4 filters?

David

David,
I'm using a combo of ND filters (large Singh-Ray NDs) and a polarizer. I still haven't seen the Vari-ND but I'm satisfied with my current setup.