To convert VDI to VMDK, you just need to use VirtualBox itself.

VBoxManage clonehd | { -format VDI|VMDK|VHD|RAW| } {-remember }

format: Allow to choose a file format for the output file different from the file format of the input file.
remember: Keep the destination image registered after it was successfully written.

Once this is done, In vmware create a new virtual machine and when setting up the disk drive, point at that file .vmdk file.

Beware, of the the disk type. In some Os changing from IDE to SCSI (or vice-versa) may cause problem. Make sure you use the proper type.