Wood – find a metal circular trim

I'll post this here although I haven't actually read it, I currently have it on order as I plan on doing some built in bookshelves for my media equipment. A friend I know who makes his own furniture suggested it: Illustrated Cabinetmaking

Both vinyl and aluminum have similar issues regarding thermal expansion. The siding must be installed with a gap to fixed objects in order for there to be room for expansion. If this is not done, the siding will expand against the obstruction. Not being able to move in that direction, it will buckle along it's length, causing unsightly bulges.

Therefor, J-channel is a common element in installation of both materials and is readily available in both aluminum and vinyl. I would consider J-channel the best solution, but it does involve more work than simply re-caulking.