The Metro, most likely.
There's stations all over the city. In case of doubt (I saw you're interested in vending, so you might have a lot of luggage) there's always taxis. Expensive, yes, but safer when it comes to transport of delicate items.
Metro Line M9 stopps at "Robespierre", rather close to the convention.
I haven't checked this myself now, perhaps a train line goes into the city and from there you can switch to Metro. I'd arrive via train, so I end up almost in the center anyway.
I am unfamiliar with the Metro system in Paris, as far as I can tell from google maps, there's a number of lines that go rather straight into certain directions, meeting in a few places. This should be where people can switch and then go another direction.
In any case I'd like to see an overview or tutorial on the Metro. Tickets for instance, how far they take you, wether or not you can board multiple lines and such.
As an example on our subways, trams and busses in Düsseldorf (germany):
We have ticket prices from A to E. A will get you around town. Limits: valid 90 minutes and only for ONE ride to your destination. Going back demands another A ticket.
B gets you to neighboring towns.
C to towns neighboring your neighbors.
D to cities beyond that.
E pretty much to the end of the punblic-transport-union-area.
The higher the ticket level, the longer it is valid. There are also day-long tickets, so if yu spent the entiere day in town, A-day-ticket is slightly more than two single A tickets in price, but valid 24 hours, if stamped at th ebeginning of the new day (which is 3am nighttime, so that you can stay out until afte rmidnight and not worry).
Now Paris is huge. Compared to our system here I'd need at least a B ticket to get from one end to the other - not counting the outskirts.
Seeing how frequent Simontheb is caring for any questions and keeping updating, I bet they'd even look up a possible route in town for us to get to the venue. After all, I'd not be the only one arriving via train, so this isn't a one-man deal. Plus, there's more people coming via plane, so you are not alone either.