Those two sentences mean essentially the same thing, but are expressing it slightly differently (but both correctly). With the adverb it means pretty directly "it is nice to meet you" where the adverb describes the action. With the noun, it means "it is a pleasure to meet you," equating the action with the noun pleasure.
Also, your username is pona. :)
I agree with LouisSepdekdu here.
As far as FredCapp describes English and not Esperanto, I agree with him here too.
I still, also, hold that the subject of this sentence is an "understood" word.
I think that's the wrong way to look at it. English requires a "dummy it" here. Esperanto does not because there is no "actual it" that is good here. It's renkonti vin that's good.
We can have infinitive subjects in English too:
- To know him is to love him.
In practice - whether we think of the subject as impersonal (NOT "implied") or as the infinitive, the structure is the same.
In Esperanto Adverbs are, indeed, used, in connection with forms of esti, as the compliment of a verbal subject.
Vin, with its predicate -n cannot be in the subject. But that's a worthy attempt to find an understood subject.
If there is any subject, it is the implied, or understood, my as in "It is my pleasure…"