i have seen a couple flies fly otu of my viv when i open it up shold i change substrate or be worried at all they look likr fruit flies
They won't hurt your geckos, but flies can be a pain to you and those living in your house. The most typical flies that are found in vivariums are fungus gnats and fruit flies. Replacing your substrate can certainly help the situation, but if you have a bioactive substrate in a naturalistic enclosure this can be a pain.
Fungus gnats look like little black flies and are in the mosquito family. They are attracted to decaying vegetation and damp environments. Many times they are lured into homes by over-watered or dying house plants. If you have live plants in your enclosure they are probably being drawn to your enclosure(s)by these and you need to make sure that the enclosure's plants are not being over-watered/over-sprayed and that all dead leaves are being trimmed away on a regular basis. Wiping rubbing alcohol on the leaves can also help as a deterrent, as well as setting up fly-paper or insect traps around (but not in) the enclosure itself. Stirring the substrate to keep things from getting stagnant can also help, since they are also attracted to the molds and fungi that live in the soil.
Fruit flies are more of a brownish/yellow color and have red eyes, but they are still very small in size. If you are getting fruit flies then you may be leaving your gecko's food in the enclosure for too long. Try replacing the food more often and make sure there are no fruits or vegetables that may be sitting out as a lure in your kitchen.