Bonvenon means "well-come", as in "it is good that you came".
You can not use "vi" because it doesn't create a proper sentence.
Saying "vi estas bonvenon" is not correct Esperanto. It literally means "You are, a good coming". You should not use the accusative (use the N ending) when using the verb "estas".
"Bonvenon" is short for a more complex sentence, something like "mi ŝatas vian bonvenon", "I like your well-coming, I like that you came well".
If something like that were your answer, you would be technically correct, but never as a reply to "thank you".
If someone says "dankon!", thank you!, you should reply with "nedankinde!", "Nothing to thank for!"
With no context, there is nothing to tell us whether this is the colloquial American expression used after someone has said thank you, or telling a visitor that you are glad they have come. Has anyone tried, "Vi estas bonvena"? Was it accepted?