Interesting conversation.
First of all, did I understand you correctly - that the store you're opening will be the THIRD vape store in Phoenix!!?!!?!?!?! Or did you mean *your* third store?? (We have about 30 stores in Greenville, SC and we have about 1/20th of Phoenix's population.)
Anyway, juice samples in decent clearos is a necessity. Being able to try juice before you buy it is the single biggest advantage B&Ms have over online sites. Having friendly employees available to answer questions, etc. is the second-biggest advantage. Play to those strengths.
A B&M can't match online prices (unless you're a big chain). Be as reasonable as you can but accept that there are some people who will always go for the absolute cheapest price. And then they will come to you with their questions. Be nice to them, it often pays off because they'll remember that you helped them out even when you didn't have to.
Percentage-wise, it's easier to make a profit on smaller things (coils, juice, tanks) over bigger things (devices).
I'm iffy on the 'comfortable place to hang out' plan. Friendly, yes, but you don't necessarily want people to hang around for hours. We've had quite a few customers tell us they don't like going to certain other stores specifically because there's always people hanging around in there and it made them feel uncomfortable.
Never badmouth another store.
I know a lot of stores have 'reward programs'. We do not (since I, personally, think they're scams and I don't like giving out my personal information). I have had customers ask about them but I just say that we believe in offering a good price to everyone all the time. Since we tend to be cheaper than our competitors, it hasn't been a problem. Also, we give out free juice fairly frequently to regular customers.
Brands to carry - obviously KangerTech, Aspire, and Innokin are the big three (at least in this area). It might be different where you are. But carry coils/accessories for other brands. We have a devoted bunch of customers that come to us because we're one of the few stores that still carries iClear 16/30 heads.
I could keep going on but it's getting late. Feel free to PM me if you have any more specific questions.