IMHO,
I don't think the VC's are set too low.
for WWIV: I think 50 centers is an appropriate win condition simply due to the vastness of the map. 50 sc's is difficult to attain. When thinking of the proportion of 50 sc's to ALL of the sc's on the map, you must consider how many of those SC's are quite out of reach. On the WWIV map, there are 34 players and surely at least 10 players will become strong enough to have establish unbreakable stalemate lines, by using the sea-zones and coastal areas effectively.
Kaner had done some work previously on making a map with more sea zones to offer greater flexibility in naval combat. If that were the case, then I would agree that maybe a larger VC would be appropriate; however, as it is now, I think 50 sc's is appropriate simply due to the fact that in most cases, getting a solo victory requires establishing a base on another continent, which is really hard to do on that map because there are a disproportionate amount of sea-zones to the sc counts of individual land-masses. For this reason, the VC should be lower than 50%+. I have never seen a player "cruise" to an easy victory on the WWIV map, unless there are some crucial nmr's or cd's, which is another story. If the game is active, it's usually pretty easy to stop a solo on the WWIV map.
As far as the viking map, I believe it is OK because it was the intention of the creator. And getting a solo on that map isn't a piece of cake either (maybe it is easier in gunboats). As Dipstick points out, it may very well alter the dynamic of the game, but don't we all like this site because of the diversity of the games and the maps? I don't think all the maps must have the goal of all playing like a standard game of Diplomacy, I think it is fine to have some with a slightly different dynamic. Besides, if you think about it, conquering nations and gaining ultimate power WAS easier in the times of the vikings than it would be in today's diplomatic and globalized world.