We are talking about two different thing here: 1- How bugs find us ?,they can smell us from around 30 feet away and then they follow CO2 that we exhale to reach us.
2- How do they find and pinpoint the blood vessels to suck our blood? this part has nothing to do with CO2 emission.
Mosquitoes don't see very well, but they zoom in like a heat-seeking missile. In the spherical arrangement of their compound eyes, blind spots separate each eye from the next one. As a result, they can't see you until they are 30 feet (10 meters) away. Even then, they have trouble distinguishing you from any object of similar size and shape: tree stump, 55-gallon drum, etc. When they are 10 feet (3 meters) away they use extremely sensitive thermal receptors on the tip of their antennae to locate blood near the surface of the skin. The range of these receptors increases threefold when the humidity is high.
My experience with bugs is that when they are out in full force the weather is hot and normally humid.In a hot and humid day covering our body with vaseline is just a recipe for heat exhaustion.