Counterbored vise base plugs allow you to use a 1/2"-13 button head cap screw to secure your vise directly to the table. The base of the counterbore sits 1/2" above the base of the vise. Add 1/2" to the depth of your T-slot, subtract 1/8-1/4", and that's the length of screw you need. Make sure the screw doesn't bottom out on the T-slot, or the vise will come loose during machining.  

Plain vise plugs prevent chips from building up the holes. 

Both types of plugs are made of low carbon steel with a black oxide coating. Plugs are secured from the bottom using a 3/4" allen. You can alternatively use 3/4" hex stock, or an appropriately sized hex nut, torqued with a 3/4" socket.   

Sold in pairs.