When you say "combine the management and vMotion VLANs on one vSwitch with 2 uplinks", are you saying the management and vMotion traffic will be on the same VLAN with the same IP subnet? (sorry for my lack of comprehension)
No, I mean you should configure the physical switch ports this vSwitch is connected to with ordinary 802.1q VLAN tagging for your separate vMotion and management network. Then you assign these 2 VLAN IDs to the respective vmkernel interfaces.
The ethernet frames will arrive with VLAN tags on the vSwitch, which will hand it to the vmkernel ports accordingly or will add the correct VLAN tag when transmitting frames. This preserves full layer 2 isolation between the 2 VLANs.
Until now your physical switch handled the whole VLAN tagging and un-tagging, you merely bridged the connections carrying a single VLAN to your host ("External Switch Tagging" in VMware lingo). This requires different vSwitches if you want to separate traffic.
I suggest you read the following to achieve a better understanding of the ESXi networking side:
http://www.vmware.com/files/pdf/virtual_networking_concepts.pdf
http://kb.vmware.com/kb/1003806
http://kb.vmware.com/kb/1004074
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