So, if I am putting down say 20 percent on a new loan for a vacation rental, my realistic goal should be "Let other people pay off the rest of my investment for me."

This is an important status for people to go for since it allows you to write off losses from rental activities on your other active income. 

When I run my own numbers theu are way off from what they claim..

I am in the market every day and can be on or off as a buyer within a few hours of you providing the address.

This was an off-market deal that my wife found through networking.

Typically, the existing note from seller would have to be paid off through the construction loan. 

Off to the next one?

While the heater has electrical components and didn't work while our electric was off or rolling, the fireplace did and saved my pipes.   

This leaves friends/relatives wanting to help you out (emotional based investing) or people unsophisticated and naive (likely disciples of a currently heavily promoted guru).When I, or any other investors (with significant capital) I know of invest, either equity or debt), we only consider deals where (1) the “sponsor”, or borrower have a significant investment of their own capital, unless they’ve owned the property many years.  

You will just want to make sure the property cashflows well enough to pay the HELOC payment or you have an exit plan to pay off the HELOC.