1. Pretty much, but there is no brick protection via bootmii or priiloader yet. As far as game play, WiiMC and stuff like that, yes they work, but if you plan on messing with wads of any type I wouldn't do that on the vWii if I were you. Thats exactly why I haven't sold my old Wii's just yet.
2. No you wont. The vWii video output is exactly the same as on the Wii. There is no upscaling.
3. No, your old drive will work just fine, provided it is externally powered. The Wii U's usb ports to not supply as much power as the old Wii's did, so the drive needs to be externally powered.
4. Using the Wii to Wii U transfer utility available thru the shop channel you can transfer all of your game saves, WiiWare and Mii's to the Wii U. Keep in mind this erases them all from the old Wii, so if you want to keep them on both consoles you will need to look into nand emulation or savegame manager gx.