The 6 months 0% interest is for any/all payments of at least $100 -- deposit, final payment and any inbetween. The 6-month period begins when you make that charge (or the first statement after).
One "quirk" -- if there are ANY credits outside of the normal billing cycle payment, that will be applied to your promotional balance. For example, you make a purchase of an item costing $100 but then decide to return that item. Instead of reducing your regular monthly balance by that $100 credit, it will be applied to the promotional balance. The same is true if you make multiple payments per month -- the first payment after your billing date will be applied to your regular balance but any additional payments are applied to the promotional balance.
I believe you must make a minimum payment towards the promotional balance each month. So if you regularly pay-off your balance each month but charge $1000 for a vacation and want to split it up, you must pay a minimal amount (maybe $25?) in addition to your regular balance each month. I have read reports from people who indicated their regular payment was applied to the promotional balance starting around month 4 or 5 if they hadn't made any payments towards that promotional balance. I have not experienced that, but then I've always paid something towards the vacation balance in addition to my regular balance.
Enjoy your vacation!