Installing a vanity unit in the bathroom with a wash basin which fits into the work top instructions say to glue it to the top is this safe or do it need to be secured to the wall

This sounds like a "drop in" china lavatory. We set them by running a ring of Dap White Tub and Tile caulk under the rim and letting it cure. The compound's adhesive and the installation will be perminant. Smooth the excess caulk from around the rim to give your job a professional look. Good luck, Tom