Update 12/12/2014: While the information below is valid that VMware does not appear to support RamDiskTFTPBlockSize above 1432 due to no support to deal with IP Fragmentation. However, it has been found that it is much better to adjust RamDiskTFTPWindowSize instead of RamDiskTFTPBlockSize to speed up TFTP (reduces amount of ACKs required and does not cause IP Fragmentation). VMware and other vendors all appear to handle this scenario perfectly. Modifying the WindowSize is easy to do from most PXE providers, including WDS (modify BCD). There is an issue changing this setting in Configuration Manager due to dynamic BCD that ConfigMgr manages itself with no native way to modify the setting (see the comments section for how to workaround this issue – thanks Patrick!). More info throughout the comments section.
Update 3/18/2016: Microsoft has released an update (currently in preview) that will now allow modifying both the WindowSize and the BlockSize without a hacked DLL. Info here: https://technet.microsoft.com/en-us/library/mt595861.aspx#BKMK_RC1603. This update is in 1603 Technical Preview and should also be apart of the next Current Branch release of Configuration Manager.
Update 7/22/2016: Microsoft has released a new Current Branch of SCCM, version 1606. I have confirmed that Current Branch version 1606 contains these modifications that were in version 1603 Preview. I have also tested the WindowSize/BlockSize registry keys and they are working great.
Note: After making changes, make sure to restart the Windows Deployment Service.