"Bảo tàng" is a verb and a noun as well, while "viện bảo tàng" is a noun only. But we usually use "bảo tàng" as a noun = "viện bảo tàng" = museum.

they are the same meaning. "bảo tàng" is often used in normal talk. " Viện bảo tàng" is used in grave case, like in news, science.

"Bảo tàng" is usually used as an intransitive verb, it means "to keep and protect precious items from the old time". This can also be used as a noun and has the similar meaning to "Viện bảo tàng", it describes a place to keep and protect precious items from the old time. However, in the Vietnamese encyclopedic dictionary, "Bảo tàng" is mentioned more often. I hope this answer is clear to you :)