A new vanity changes how you feel every morning. It hides the clutter and updates a room that usually gets ignored until something leaks. The best time to do this is during the winter holiday sales when the big retailers drop prices to clear out floor models for the new year.
The physical swap usually takes a day for a professional or a full weekend if you do it yourself. Even for a simple job like this, ask three different plumbers for a quote. Prices for labor can vary wildly depending on how much they want the work during the slow season.
Keep in mind that once those 18 months are up, any remaining balance will start gathering interest at the regular rate. This window is a tool for planning your cash flow, but the bill still needs to be paid in full before the time runs out.