Hi I have some frozen tomatoes and I would like to blend them in my Vita mix and make sauce. My question is can I re-freeze the sauce once it's been cooked.
Probably. Depends on your sauce; some sauces do not tolerate freezing (e.g., they "break"). The fact that your tomatoes were frozen at one point doesn't matter.
Mostly warnings about not re-freezing foods are due to quality loss. For example, each time you freeze a vegetable, it will turn closer to mush.
Safety warnings are primarily about quick thawing (e.g., in the microwave) where the food may get into the danger zone (between 4°C–60°C/40°F–140°F); the recommendation is to cook it before freezing (if you thaw in the fridge, this is avoided). You've already got a cook step between the thaw/freeze, so even with a quick thaw method, there aren't any extra safety concerns.
I frequently freeze for up to 6 months various homemade tomato based pasta sauces, bbq sauces and hot sauces and have not had any trouble. Just thaw and make sure you mix well.
The fact that any raw ingredients were frozen is irrelevant for the sauce you intend to freeze. It is 100% safe to freeze a cooked sauce, though the quality of taste may be affected.
You can test it first if you like, put part in the freezer and keep the rest refrigerated. Thaw the frozen sample the next day and if it is still acceptable add the rest to the freezer. If not you may consider different options for storing the sauce.