I'm looking at purchasing a small multifamily (2plex, 3plex or 4plex) as an owner occupied using a VA loan.

Yes, you can do a sub-to on a VA loan.

My father served in the Navy for 4 years - Can I do a joint loan with him and utilize the VA loan options? 

If you have owned the properties for a while, take a look at appreciation and loan pay down.

This offer would also give us enough capital to finish the rehab through a construction loan and draws.However, we are also not opposed to having another team come in and complete the job if we were to find the additional 500k.Just looking for guidance or anyone who has had experience with this or hard money companies that can offer the full amount to refinance this current loan.It's a great property with a lot of upside but unfortunately was put in a difficult situation.

I just learned this today, VA loans can be assumed by non-military buyers!

Those are healthy rates for a investment property with a larger loan amount.

Would only need the loan for 6 months. 

I'm still at the very beginning, so I'm learning more about the different kinds of loans out there, as well as refinancing after doing value adds.

If you have 2-4 unit multifamily homes in that area it can help afford a higher loan amount as well as cover some or all of the mortgage payment.