SQL Server on SAN, same LUN: one logical drive vs multiple
In your question you say &quot;one LUN assigned to a SQL Server VM&quot; - what you want is one LUN assigned to the hypervisor machine that can have multiple virtual machine disks for the SQL Server stored on it.  This allows the VM to use a much larger number of threads to access the disk.
In order to get the best queue depth out of VMware, specifically, you'll need to use as many separate virtual controllers, and drives as possible.  Make sure you use the para-virtualized controller, if at all possible.  Refer to this blog from VMware.
I'd put tempdb on 2 controllers (with 4 files, spread over the two controllers/LUNs).
I'd put data on a controller, and tempdb on a controller.
Virtual controllers have their own queues (just like hardware); typically at most 32.  Pretty clearly, 32 is not a lot, so having more controllers actually increases the effective queue depth.
Clearly, if you're going after performance, there are also a lot of other settings, such as CPU and Memory reservations.
The standard warning applies here; do what you have tested as being the best solution for your system.  Since we have no idea about your actual load, I/O requirements, etc, etc, it's pretty hard for us to give you a hard-and-fast rule to follow for every installation.

VMware whitepaper on pushing I/O performance is here (warning, PDF).