A lot of games with vchat have one or more anti-spam features.
1. You can't use Vchat again until your previous message finishes.
2. A disable feature, for use on larger servers.
3. Overlapping messages only play the most recent.
4. Can't use vchat again for a specified amount of time after your previous. (See anti-flood QVMs)
5. Muting a player disables their vchat privileges.
6. Other, sneakier stuff most users wouldn't notice.