While you do need a decent mic to pick up things well, remember that VA is dependent on Windows voice recognition, which can be hit or miss even with a high dollar microphone. Be sure to train it in Windows a number of times, and if you're doing VA macros yourself, remember that it works basically by looking at the matches it gets through Windows and comparing it with the command words. What words and how you combine them can be an equal or more important thing that how well Windows picks up your words.
An example, the PREFIX/SUFFIX combination commands trigger better than the single words by themselves, because while a match of only say 70% for each word might happen in bad listening conditions, put together that's a match better than 70% in VA logic, so it triggers.
When I started playing the game and had gotten VA along with ED, I got a cheap SADES headset. Most of my commands, especially the frequent ones usually hit at 90-100%, which could have been both the choice of word(s) as well as Windows learning through repetition. And through trial and error I have had some commands that I discarded and found better words for due to them not working well.
So, get a good headset for a lot of reasons, but remember it's just one part of the whole thing.