VMPK allows playing chords with 3 notes or more - except when using the note Ab or G#. On a standard QWERTY keyboard, it happens when using the keys H and 6, for instance. It plays only 1 or 2 notes, even when pressing 3 keys. You cannot play the chord for E, for instance.
I can reproduce this bug with any instrument (program), octave, transposition or operating system. It happens on Linux and FreeBSD as well.