So, what is actually playable on the second screen?

One featureiwas really looking forward to was using the tablet to play VC and some wii games, as I was a late adopter. I've read that VC games purchased THROUGH THE WII are not compatible. So, what can you play on the tablet?Heirren

Well Wii games where NEVER going to be compatible, as they would still require the Wii Remote (+ Nunchuck), with the odd exception of games such as SSBB, Xenoblade Chronicles and Monster Hunter Tri. But yeah, it's stupid that VC games bought on the Wii won't be compatible.
As for what will. I'm pretty sure, New Super Mario Bros. U, Rayman Legends, Black Ops 2, Sonic & All-Stars Racing Transformed and Mass Effect 3 have been confirmed to be playable just on the GamePad, as will any VC games bought on WiiU (i'm assuming this with them only saying ones bought on Wii won't work), but ZombiU cannot because of heavy use of the GamePad for other features.
Overall it's up to the developers what games are playable on the Gamepad exclusively, if they want the GamePad to be a core mechanic of there game, it's likely that it won't use this feature, but titles that don't rely heavly on the Gamepad probably will IF the devs take the time to do it.

wait a minute, i thought there was a switch for using the gamepad only , or the tv and im pretty sure its for wii u games only

For the VC games I can pretty much guarantee that they will be at some point in the future be playable on the game pad. They wont be playable now, but maybe in a firmware up date a year or two from now Nintendo will figure out how to implement that.