Should I use sand to help loosen up my veggie garden soil?

If your soil is the gummy clay that Texas is famous for, adding sand will simply make it gummy, sandy, clay. You need to add humus. Compost, well rotted farm manure, rotted leaves, peat or forest humus. Even perlite/vermiculite, while having no nutrients, will improve aeration and drainage. Tilling the amendments in would be best but you can work it in around the existing plants by hand for now. Even a thick layer on top the ground will be better than nothing and in the fall or next spring use a tiller or spade to work it in deep. Composts and manures altering pH? Both will neutralize pH, which is what you want for vegetables.