Yes, Vista Service Pack 1 is required, not just recommended, before installing Service Pack 2. This is unusual, if I am not mistaken, in that previous major workstation operating system (i.e. XP) service packs contained all previous service packs or patches.
Also note that this service pack applies to both Windows Server 2008 and Vista. This speaks to the similarities between the 2 architectures, despite their very different roles.
From the Microsoft FAQ on SP 2:
"Why does the SP2 installation require SP1 be installed first?
There are a number of reasons for this, the primary one being the overall size of the standalone package for SP1. Having a cumulative SP2 that includes the updates from SP1 would have made the size too large. Furthermore, because SP2 is a single-servicing model, and since Windows Server already includes SP1, a cumulative SP2 would have meant a needlessly larger file for Server customers. Finally, in the consumer space the majority of customers have already installed SP1 over WU or will be doing so before SP2, so incorporating SP1 into a cumulative SP2 would take unnecessary time and bandwidth."
SP 2 FAQ Link..