1. Run 32-bit 6u21 JDK installer on 64-bit Vista.
2. In Custom Setup window, JavaDB is not seen in the list of available features.

The issue is reproducible for 32-bit as well as 64-bit installers on 64-bit OS.
Tested on OS: 64-bit Vista and 64-bit XP.
Tested with: 6u21 b07 and 6u20 b02 installers.

32-bit installers ran fine on 32-bit Vista/XP for 6u21/6u20.
As per Knut A. testing,

Windows 7 - 64 bit: Neither the 32-bit installer nor the 64-bit
installer offered the choice to install Java DB.

Windows 2008 - 64 bit: Same result as Windows 7 - 64 bit.