does it mean that US acting accord to Vienna Convention’s rules, rather than other(signed but not ratified one) until NOW?
Considers means thinks and accepts here.
Def. 2. To think or deem to be; regard as: considered his friend a liberal on most issues
The US thinks that most of the rules represent already standard (customary international) law. Informally (however), the US takes the rules as universally accepted. The US considers the rules a fact of life, if not a strict treaty.
Thus, since the United States considers most of the Convention’s rules as representing customary international law, the US has signed but "has not yet ratified" it, and is in no rush to ratify it soon.