I agree very much with @Neil Henderson that unless a real estate portfolio is diversified across asset class, geography and operator then it isn’t fully diversified.However I would add two more things: strategy and vintage year.

I am a huge fan of the updated vintage look in my houses but for a flip I would probably go forced air due to modern comfort, but it depends on what buyers want/ are willing to pay for.

It's been a real treat to ride my vintage mc out to jobs to do and meet interesting people/situations! 

I bought a few doors for $11.95 (instead of buying the same-sized door for $80 with frame at Lowe's - plus this vintage door isn't going to break if some angry tenant punches it).I also buy vintage glass light shades there - they look freaking amazing in a rental - often better than the newer ones available at Lowe's. 

MOST of the AL wiring we see here locally are in mobile homes of older vintage. 

We are still finding opportunities in the large apartment communities where we can add value to vintage properties built in the 1980s.  

In our older Queen Anne, Second Empire and Victorian vintage properties trim is frequently finished with shellac (or since coated w/ urethane), but where it is painted, the period calls for high-gloss.

I like building new, but we sell new, keep vintage rentals.

But, I'm glad vinyl siding is not used, as I do love to see the authenticity and beauty of vintage home brought back to life.

In Chicago there are many vintage units that are in high demand and everyone uses window units.