My vertical composite siding is starting to pillow outwards on the bottoms. See LP siding.
How can I seal it for a few years until I replace it with vinyl siding?
Here is a photo from the web.
First I would determine how old the siding is. If made in the last 20 years it's probably covered by manufacturer warranty and replacing it with vinyl would be a step backwards.
Also lap siding isn't supposed to seal on the bottom. It's supposed to allow any moisture or condensation that happens to get under the boards to drain downwards. So you should not try to seal the bottom edge of the boards, at least not with anything that is impermeable to moisture.
If the house's sheathing isn't wrapped in something like Tyvek then significant gaps at the bottom of lap boards could give an opening for pests. If you're having pest problems you might just address those.
There really is no repair once hardboard starts to decay. You could paint it to prevent some future moisture intrusion, but it won't make the siding more solid.
A good quality latex paint will add a bit of life to it. Whether that's worth the cost and effort is debatable. I'd replace any seriously rotten boards and ignore the rest until you re-side.