After boiling and draining the rice vermicelli or pho noodles they always stick to a big lump. How do I prevent this?
You have bascially two options:
Bring water to a boil. Put noodles into the pot and wait until noodles are done. In my experience this methods is quite difficult to handle because you can't be really certain about the cooking time (except you have done this often enough) since overcooking leads pretty fast to mush. I have the impression that thick noodles also tend to be undercooked in the middle while the outside is already too soft.
Unlike italian pasta surface starch is not desired since it causes an unpleasant slimy mouthfeel especially on rice vermicelli. Rinse the noodles thoroughly to remove the starch. The easiest way remove the starch is to dump the noodles into a clean water bath and then agitate the whole batch until the water doesn't get more cloudy. Shake the noodles to remove excess water.
Soak the rice noodles in warm water until the noodles are soft (one hour as a guideline, YMMV). Cold water is okay, you will have to soak them for a longer time. The noodles will not be as soft / chewy as cooked but rather can be bent to a certain extent until they break. When soft, remove the noodles from the water. (Optional: You can store the hydrated noodles in a closed container in a cool place until you need them.)
If the noodles end up in a soup or in cold dishes: Cook them for about 15 seconds (just as a guideline; duration might differ) in boiling water (not broth!). The noodles are done if they don't break if bent (maybe leave the noodles in the water for a few more seconds according to your taste). Don't overcrowd the pot. If you have a small pot (~20cm diameter) cook only one portion per batch. I recommend you to use a blanching colander if you are going to make more than one batch of noodles. Otherwise you will have mushy bits of noodles floating in the water. If you don't use the noodles immediately (like in cold dishes), rinse the noodles thoroughly. Optionally, you can oil the noodles in addition.
If you are going to stir-fry the noodles: Just put the soaked noodles directly into the hot pan and stir. Don't add too less oil (or sauce) or you'll end with a ball of noodles (I least I do, my parents do better).