This is going to be a bit weird when you're used to interacting with the universe at a macroscopic level, but the vacuum is actually more like a cosmic bank account. On balance it is at '0', but locally there can be small 'offenses' which create pairs of particles that spontaneously annihilate (again, in pairs) before anybody gets a chance to notice.
As long as the books balance (particles and anti-particles created in equal proportions) the universe does not seem to care, it even does not care if these particles are in existence for a longer period of time, or are separated by distances large enough that you can notice them.
This also apparently is one of the more interesting pieces of the puzzle about where all this stuff came from in the first place and has been used to postulate the existence of an 'anti-universe'.
I am not a physicist either, but your concept of vacuum does need updating.
The idea of matter and antimatter particles popping up out of vacuum has been around for some time, and appears to be the driving force behind black hole evaporation (hawking radiation) .
http://en.wikipedia.org/wiki/Hawking_radiation
Although a vacuum is devoid of physical matter, it's not actually empty. In fact a vacuum is host to a seething "ocean" of virtual pair-production (a particle and it's anti-matter counterpart locked in a feynman loop) giving rise to the "Vacuum Energy" http://en.wikipedia.org/wiki/Vacuum_energy, potentially a vast untapped reservoir of essentially limitless power - if it can be harnessed :)
Please correct any misunderstandings I may have in the following:
* Vacuum has no mass (gravitational attraction).
* Normal matter has mass.
* Vacuum is composed of matter and antimatter.
* Therefore, antimatter has negative mass (negative gravitational attraction).
Its tricky to answer, as I am limited to browsing wikipedia pages on these topics and no more knowledgeable than you are on the topic :-). One obvious alternative to your line of thinking is that the matter+anti-matter that is created in a vacuum is not 'normal' (using your position on normal matter), and has no mass.
It seems to have something to do with normally existing only for a very short time, and affecting the universe only on a very short range that these virtual particles get away with appearing to not having a mass from the point of view of the rest of the universe.
http://en.wikipedia.org/wiki/Virtual_particles
I'm not a physicist, but this sounds wrong. Vacuum doesn't have "matter and antimatter components", it's vacuum.