Vinny had a great time with Grandma and Grandpa while we were in Austin. Grandpa took Vinny to see his (Grandpa's) mother, who lives in Indiana, and Vinny entertained himself by pointing out all the bridges over the interstate. He also slept like a champ, crying for a maximum of 45 seconds each night before going to sleep.
So we tried just putting him in his room to sleep last night, and instead of carrying on and whacking the door, he cried for a couple of minutes before going to sleep. So maybe he's reached a new developmental level and is no longer afraid of sleeping alone. Either that, or he couldn't play Grandma and Grandpa like he could his parents, and has gotten out of the habit.