If i make prawn stock for a paella, can i use the vegatables from that stcok in the dish, or will they be tasteless?

Yes, you can use the veg they should have the flavour from the 'prawns'. 

It depends on how long the vegetables have been used for the prawn stock.  If the vegetables are not mushy and has flavor, you can use it in the paella.  

The veggies will probably be mushy, but they should be full of flavor due to being in the stock with all the other flavorings.

Typically after making stock the ingredients of the stock have given up their goodness into the stock or broth and are flavorless and mushy.  I will almost always toss the solids when my stock is done.

The exeptions to that are when I am making a meat stock, I will often go through the solids by hand and separate out any meat pieces to feed to my dogs and cats.  Sometimes I also give them the spent carrots, which they love.

The vegetables have done their job and the flavors have leaked into the stock. Like making a chicken soup, just toss them out and use fresh