Auto-keywords vocabularies:

1.are all the terms in the vocabullary added to keywords or is it only the user selected term (or terms) that will be added ?

Only the terms the user has selected for the specific node.

Example:

1. Vocabulary "foo" has 3 terms: "one", "two", "three" and is selected as auto-keywords vocabulary;
2. Global keywords is set to "global keyword";
3. The node 42 has set "node keyword" as keyword meta tag;
4. The node 42 has 2 terms of vocabulary "foo" selected: "one" and "three";
5. If the user goes to node/42, KEYWORDS will contain: "node keyword,one,three,global keyword".

2.how will meta tags act if there are multiple vocabularies for a content type?

All terms of all vocabularies (that are auto-keyword vocabularies) that are assigned to a node will be added to KEYWORDS.

Thats wonderful.Thank you Jacques