I'm always confused when to use von, vom or aus in a sentence, such as "Warm greetings from Cape Town", so which do I use in this case?
Ev. Kounis' comment is correct:
"Aus" is used when you are "in" something while "von" is used when you are "bei", "über", "auf" ... something.
For male and neutral words with article "von dem" becomes "vom". (There is no short form of "von der", "aus dem" or "aus der".)
City and country names typically have no article and you are "in" a city or "in" a country. Therefore "aus" is correct.
Grüße aus England (England is a country and you are in a country.)
Grüße aus dem Mustermannhaus (You are in this house when writing this message.)
Grüße vom Mustermannhaus (You are near this house when writing this message.)
In addition to Martin Rosenau's answer, I'd like to add:
The fact that you can greet aus locations you are in, would make countries, cities, houses, etc locations you greet aus.
Grüße aus dem Kölner Dom (when you are inside)
Grüße aus England
Grüße aus Kapstadt
Grüße aus dem Haus meiner Großeltern
If you are on or at that place you greet from, you greet von, like islands, sights, etc.:
Grüße vom Kölner Dom (when you are in front of it or on one of the towers)
Grüße von Koh Samui
Grüße vom Kap der guten Hoffnung
Grüße vom Haus(dach) meiner Großeltern
This is just a rough generalization, but it will be a good advice for most cases. Please feel free to add edge cases to the comments for discussion.