I'm on Vista 64-bit, and I need to have VMWare Workstation run as a 32-bit process for reasons outside of the scope of this problem. Right now when I run VMWare, it starts it as a 64-bit process, and I have no idea how to make it run as a 32-bit process instead.
The guest OS is Windows XP (32-bit) if that matters, but I doubt it does.
OK, since we've discussed in the comments what it is you are really trying to do. I will answer the question.
First, your original question: "How do you run a 32-bit vmware host on 64-bit windows?"
From looking at the VMware site and documentation, there is no indication that this is possible. If you really need to do this, run it on a 32-bit version of windows.
Now on to what you are actually trying to do: You have indicated that you have tools written which are used for controlling the VMware processes. The proper API to do this is the VIX API or using the vmrun application (the later is more for scripting from a shell). I am assuming VIX. If you are using this. I imagine that ensuring you have the latest copy of the SDK and recompiling your apps should allow them to work with the 64-bit version of VMware.
EDIT: going to the VIX SDK download page shows "VIX API for Window 32-bit and 64-bit". This indicates that if you use that version of the SDK, you should be good to go with a 64-bit VMWare.
Your best bet would be to ask over at the VMware Forums where you can get an answer direct from the developers.
I ran into a similar problem. I was running VMWare on 64bit Vista but I could only get VMWare to load 32 bit OSs. It turns out on a Dell you have to go into the BIOS to set up "virtualization" to be either 32 or 64 bit. Not sure what kind of hardware you are on but check the BIOS to see if you have the same setting. It doesn't affect the base OS at all, just the virtual ones.