I would say at this post generating more cash flow to live and invest with is the primary goal.

I'm seeking strategies for assuming someone else's mortgage as a primary residence. 

The VA loan requires you to live in the property as your primary residence which is defined as at least 51% of the time.

If you are buying this property as a secondary home, a secondary home loan tends to be accompanied by a similar interest rate to that of a primary residence and could result in as little as 10% down.

You can take advantage of Downpayment assistance loans if this is going to be your primary residence.

as you were - what I meant was VA loans are for primary residences/properties in “move-in ready” condition, with very few exceptions.

I thought after a year of being in my primary with a VA loan I could qualify for another VA and buy another primary and keep the other mortgage as a rental.

Yes you can as long as your father will live in the home as his primary residence and you put enough down to cover your portion of the loan that VA will not guarantee.Here are the VA guidelines for calculating the down payment.VA calculates the guaranty as described in the table below.Step Action1 Divide the total loan amount by the number of borrowers.2 Multiply the result by the number of veteran-borrowers who will beusing entitlement on the loan.There is usually only one veteran borrower, in which case the resultof this Step is the same as the result of Step 1.3 Calculate the maximum potential guaranty on the portion of the loanarrived at in Step 2 (as if that portion was the total loan).Use the maximum guaranty table in section 4 of chapter 3 of thishandbook.4 VA will guarantee the lesser of:• the maximum potential guaranty amount arrived at in Step 3, or• the combined available entitlement of all veteran-borrowers.5 VA makes a charge to the veteran-borrower’s available entitlementin the amount of the guaranty.If more than one veteran is involved, VA divides the entitlementcharge equally between them if possible.

My provisional strategy is to put a room in a property up for rent while also living in it as my primary residence for a year.

Or does it needs to be primary residence if we assume the loan