Pagona vitticeps is a desert dwelling creature. Standing water is not common in their natural environment. Thus, they have not developed any affinity to
drinking from such sources.
Your vegetable salad should provide nearly all of the water your beardies need. I even will put about an 1/8" of water in the salad to keep it
hydrated and so that they get plenty of moisture.
If you really want to get your dragon
drinking from a water dish, try trickling it for them. I mean by this, when refilling or changing their water, instead of putting a full dish down, put it down empty and slowly fill it (making a running creek sound). This has peaked my dragons' interest and they used to drink before I took the dishes away.
A 1mL dropper has also been very effective for me. The dragons drink easily from me. Though this may be due to my hand feeding a vegetable puree (about like organic baby food) when they are young.