Vacancy is a thoretical concept. 

Live in a brand new, uniquely designed open-concept floor plan full of upscale features floor to ceiling.

That way they don't waste a plane ticket, only to get here and find the house under contract.It's a weird concept, but plenty of people are willing to do it.If your market is that hot and your contracts allow you a way to back out...that might be your option. 

In halls of Academia there is a concept called Blue Balls( there is the Puppy you want Dearly, but you cant pet it-something like that). 

Another option is to consider strategies other than LTR that may increase the gross revenue like renting a unit or two by the room to college students or perhaps testing a corporate furnished rental concept.

I just briefly looking up the Smith Maneuver, a fascinating concept, any downfalls?

I'll let actual investors speak to your questions from real life experience, but wanted to help you approach the concept from the right starting point.  

It was his stuff where I first seen the concept that a house (buy and hold) has all kinds of ways to make money from tax benefits, to appreciation, to equity build up as the tenant pays it off and of course cash flow.

If you mix the two concepts, it can monkey with your cashflow pretty badly.

It doesn't mean I ever UNDERSTOOD math concepts though.