There seems little point in splitting an image file unless you're creating it on a FAT32 volume or network share that has file size restrictions.
VMware has this to say:
Select Split virtual disk into multiple files if the virtual disk is stored on a file system that has a file size limitation.
Another reference has this to say:
Choosing to split VMDK into 2GB files has quite no impact on performance.
Anyway, the largest disk image file I've currently got (on my NTFS-formatted portable USB hard drive) is ~150GB. I've even backed it up using 7-zip to a network share overnight. No problems at all.
Performance-wise, seriously, for what you're doing you simply won't notice the difference between single and split images. I imagine a perceivable difference would start to appear with images into the 100s of GB in size. Still, if you want to go with what is technically the most performant, then that's a single monolithic image file which is what you're doing already. You're all good ;-)