There are a few Guerlain boutiques in Paris, but I believe the one you're looking for OP is at 68 Ave des Champs Elysees as someone else mentioned before, which is essentially the flagship.
The one that MSPEconomist is referring to is at 356 rue Saint Honore, closer to the Opera/Louvre.
If the Guerlain you're looking for is truly near rue de la Paix/Place Vendome (or if you have time to be in the area), I recommend Le Petit Vendome which does fantastic take-out sandwiches (no sit down) for around 6 to 7 euro. It is just around the corner from the nearby Park Hyatt. If you are sitting down (takes longer, can't be rushed), the charcuterie and tartare is superb.
The distance is not that far between Champs Elysees and Place Vendome - you could walk (30 mins), take RER (20 min), or maybe you're brave enough to pick up one of the electric scooters.