Founders, what's your take on vacation policy? 

"Unlimited" moreso means "unlimited within reason", which is ambiguous and often leads employees to take less vacation. 

Fixed limit that increases with seniority?

cma.xyz. Founder /paragraph, ceo /mirror, previously at Google & Coinbase 

Only experience is with early stage and it was "whatever you want". Longest somebody took off at once was 2 weeks. 
And "closed" xmas through new years (just asked people esp devs to watch for outages)

founder @ paperchain.com. smoker's club. always was, always will be aboriginal land.

I imagine would need to be far more codified as company grows.