Question: "What does 'plain vanilla' mean?" And the answer? It means 'ordinary' 'basic' or 'standard'. It's an idiom based on ice cream. In many countries, 'plain' ice cream is whitish and vanilla-flavoured. If you don't ask for a more exciting or exotic flavour, what you get is 'plain vanilla'. So in this case, 'plain vanilla market monitoring' is the basic ordinary service, as opposed to the more advanced, specialised, and customised services which this company also offers.

In terms of taste or flavour, i would expect plain vanilla to mean only flavoured with vanilla.