My vacation rental performed really well during tourist season, then poorly for the other 6-7 months.

You must occupy the residence within 60 days of closing and live in it as your primary residence for 12 months before renting out other units.Do this several times and you will have 3-4 performing (hopefully) 4 plexs (or tri or duplex) and then you can look at doing STRs.It would create a great base for you to work towards early retirement. 

For example a DSCR (debt service coverage ratio) type loan looks at how a rental performs to determine how it qualifies for a mortgage.

but no one is promising you success, they're just performing a set of services.  

My property performance is way below what AirDNA would estimate so I am trying to do some investigation on my own.

Generally speaking self-performing construction can be wise if you are an experienced builder.

Good morning everyone,Curious to see if someone could help with the following question:If I perform a cost segregation study for a short-term rental property purchased the previous year, would I amend the return for the previous year or adjust in the year of the cost segregation study?

I'd have to plan around that in case I have to front the rent due to poor performing rentals. 

They will work with the buyer to re-set the loan and terms because when they foreclose and bid the property in for their loan amount which they mostly do around here, they have to recognize the loss as a non performing loan which has a multiplier effect on their lending base.

The biggest "risk" is them performing, because at end of term that means they complete purchase of property.