If you were given extra time previously, it is likely you were on vacation less than 24 hours.
Although not stated here http://support.chess.com/Knowledgebase/Article/View/26/0/how-does-vacation-work--how-much-time-do-i-get you received the remainder of the minimum 24 hours vacation charge as extra game time, if you came back before 24 hours.
Either you were on vacation more than 24 hours or that unwritten policy was changed.
I took more than a day vacation. But why would it be different if I take 30 hours vacation? Shouldn't I get charged two days vacation but get the remaining hours added to my time to make a move? Why would it make a difference whether the vacation time is less or more than one day? The effect is the same. If I have several games with little time left I would immediately go back on vacation except that at that point now I should immediately resume playing as now I would get my hours added to it. Do I have this about right?
Probably has something to do with the automatic vacation settings. Doesn't it automatically put you on vacation when time gets low enough? So while you pondered the third game, the automatic vacation turned on due to time requirements being met by the auto-vacation settings. That's my best understanding of the situation...
I took more than a day vacation. But why would it be different if I take 30 hours vacation? Shouldn't I get charged two days vacation but get the remaining hours added to my time to make a move? Why would it make a difference whether the vacation time is less or more than one day? The effect is the same. If I have several games with little time left I would immediately go back on vacation except that at that point now I should immediately resume playing as now I would get my hours added to it. Do I have this about right?
You are working on a false premise of what the extra time is for.
The One-Day Minimum
It's important not to expend vacation time frivolously or abusively. To discourage these behaviors, we have implemented a one-day minimum on vacation length:
Just leave vacation on until you are ready to make enough rapid-fire moves to get a comfortable cushion.
Wasn't the vacation routine changed prior to the layout change that when a player resumes his games time would be added so he doesn't go back right on vacation if he has several games where the player is short on time? I just had this problem where I had three games where I was low on time. While I was able to make two moves I was returned to vacation while I ponder my third game. Is this something that will be fixed or did we revert back to the old system?