"vom" = "von dem". This is not possible. I'll try to give an explanation why, but I'm not too sure about that either.
"von" would refer to a movement from one direction. If you say "der Westen" you are using it as an abstract object, just like a country.
For a country there are rarely cases in which you would say things like "Ich komme von Deutschland". This would only be used, if you are talking about a specific route you have taken, like "How did you get to Germany? From Austria or from Switzerland?" - "Wie bist du nach Deutschland gekommen? Von Österreich (aus) oder von der Schweiz (aus)?"
"Der Westen" is not used in a context like that.
This is particularly interesting as in China people generally think that all europeans are "Westerners", which carries a us-verses-them value judgement. But I read a book in which the author said that, historically speaking (Prussia times I believe), the moment Germans hear the word, they would think of the English, as they are the people in the west considered by the German language at the time a potential rival.
This is Off-Topic, but well: In Germany there is a quite clear distinction between "western" and "eastern" countries.
Up until 1990 the border between "the west" and "the east" took place between fromer West Germany and East Germany. Everything associated with the Soviet Union was considered "East" (including Poland, Czechoslovakia, Jugoslavia...), while everything associated with the USA (including France, Italy, Spain, ...) was considered "West".
The association is still prevailing, although some of the former "eastern" countries are now considered to be more influenced by "western" culture (and therefore part of the "west"). This includes e.g. former East Germany, Poland, Czechia.
Well, considering what you just said about the German concept of "eastern" and "western", I on the contrary believe that this is the most important topic, since this involves the fundamental definition of "words".
And regarding to the Germany reunion, about that we could not feel more affectionately. See-http://en.wikipedia.org/wiki/Tiananmen_Square_protests_of_1989
And I am a bit curious about the status of Turkey in terms of this dicotomy. Since the two countries have quiet a complicated history and a story going on. Thanks~