With MBR partitioning, you can have 3 "primary" partitions and 1 "extended" partition.. an extended partition is essentially a link to an additional partition table that again can have an additional 3 primary, 1 extended partition. (
..this can keep going for quite a bit unfortunately.).
You already have 4 primary partitions, another variation.. thus you cannot create any extended partitions..
This can be quite confusing, and perhaps a GUI partitioning tool might be able to simplify it.. but the gist of what I'm telling you is that you'll need to repartition your disk, having 3 MBR partitions for Vista and for your laptops utility partition.. and the last partition being extended, afterword you can add a partition for OpenBSD and one for the EXT2/Linux stuff.
OpenBSD 4.5 supports being installed on extended partitions now, and
fdisk(8) has supported manipulation of such tables for some time as well.
Please search for the "COMMAND MODE" section in that man page for more information about switching between extended table views.
OpenBSD creates "fictitious" disklabel entries for foreign partitions it finds.. but I have not confirmed that it recurses through extended partition tables.
Can anyone comment on this? I never install more then one operating system on my computer.