My edge 840 does not have the Virtual Partner feature to select. How do I get the VP feature?

Virtual partner shows up when you have a course loaded. I believe the consensus is that it's not very helpful and in my experience it can sometimes lead to premature turns. It goes at a constant rate regardless of wind, traffic, or gradient. 

As already mentioned, you need to be riding a course. From within Garmin Connect you can edit the course and set a pace for the VP to travel at.
You can specify data fields on your Edge 840 such such as Distance Ahead or Time Ahead. I quite like it and use it as a mini competition with myself 

6978990 said:

I believe the consensus is that it's not very helpful

Given that it's been a feature for every Edge over a decade, it seems quite a few people find it useful.

There are some people who would like to be able to turn it off (that's reasonable).

6978990 said:

It goes at a constant rate regardless of wind, traffic, or gradient.

The VP moves at a rate indicated by the timestamps in the track file. Route planners use a constant speed to generate the timestamsp.

The "original" intent of the VP was to use a recorded ride (where the speed varies).