I wouldn't put guildwars in the MMO category tbh, its so heavily instanced that it is essentially a normal multiplayer game for all practical purposes. (there can be lots of players in the towns but thats really just a fancy lobby for what is essentially a normal MORPG (highest number of players in a game in guildwars is 16 or 24(alliance battles involve 24 players but seem to be played as a series of 4vs4 games) which is hardly massive by any standard))
Yes, I actually was in doubt of whether Guild Wars qualified myself, but I think it does qualify as massive when looking at it from a content perspective
The first M traditionally refers to the scale of the multiplayer itself(Massivly Multiplayer) not the size of the game world (MMOs tend to be large content wise in order to get players to spread out since large crowds will inevitably chew up insane amounts of bandwidth), it is possible to make a MMO with a small game world if you restrict direct player->player interactions (Which is what most web based MMOs do). (Allthough some webbased "MMOs" restrict player->player interactions to such a point that one can debate weather it is a multiplayer game or a singleplayer game with some social features tacked on).
As for the VB6 vs VB.Net debate, it really doesn't matter in the end, VB6 is an awful language and i'd personally stay away from it but any language that can open a socket and display graphics can be used for a MMO client. (For the server you just have to be able to open a socket (Other features can make your life alot easier though))