I reckon this problem can be down to various things, but I thought the solution that worked for me might be worth sharing.
I was using a cheap rice cooker, was washing the rice first, using the correct rice/water ratio, and wasn't over-filling or under-filling the cooker, but still it made a mess of the counter.
What I observed was the mess wasn't coming from the vent hole, but that the steam was lifting the lid, and starchy water would coat the rim of the cooker, when the lid dropped, it spattered the water - a tiny spray, but happening constantly, causing the mess.
I figured the rice-cooker lid wasn't heavy enough, so I tried a cook with a large heavy mug inverted over the handle, to weigh it down. Immediately I got a good strong jet of (clean) steam out of the vent, which I wasn't getting before. The lid no longer rattled, and there was no mess at all, not a drop.
I've since done the same a few times, and haven't had a mess.
Of course, it depends on exactly how your cooker is making a mess, but this worked for me, so it might be worth a try; especially if you are aware that your lid is rattling all through the cook.