Since I upgraded my whole system and moved to Systemd and Pulseaudio I can't get input sound from my USB video capture device (dmesg command says it's "em28xx", using V4L2 driver). Pulseaudio is configured well, I tried Audacity and Skype, so it can play and record audio from my internal device but not from external. I've used pavucontrol and PulseAudio recognises device Em28xx so aparently there is no problem with it. VLC can select this device as hw(2,0) but final capture doesn't have any sound.
I'm slightly confused by your post. Is the external mic not working with any audio program, or just VLC?
Check your sound levels to ensure that the external mic input is turned up. Alsamixer makes this easy to do and test. See here in the ALSA wiki for more details.
Machine: Gateway NV55C Dual core Intel Pentium CPU P6100