Gee. That's really a hard question. Vez is more like an occurrence. Tiempo as time is something you measure, and you usually use it with es. On an advanced Spanish test, they'll sneak in "¿Qué tiempo hace?" = " What's the weather like? But "hace tiempo" = "It's been a long time". If you're talking about the time and weather a la misma vez (at the same time) better stick to "la hora" for time, and "el clima" for weather. (And yes, it is el clima).