If grub is doing the booting, it doesn't matter what you do with EasyBCD to the BCD. That's used by the MS bootmgr and has nothing to do with grub at all.
However, your post is a little confusing.
BCD entries are in the BCD and have nothing to do with the MBR.
"Created a new entry with a new name and wrote it to the MBR"
What did you do ?
If you add a BCD entry, you click "add entry" on that page and it's done.
If you then go to BCD deployment, select "install W7 boot loader" and click "write MBR"
then you are replacing the grub boot with the Vista/7 boot and refreshing the boot information in an infinite loop of adding and removing the BCD entry.
"Write MBR" should only ever be used if you lose the MS boot and you want it back (e.g. Install XP after Vista/7, or Install Linux without preventing it from taking control of the MBR) or if you want to go in the other direction and abandon Vista/7 and return to an XP-only PC, then you would use it with the other option (Install XP......) selected.
In either case it's a one-time only option, to change the very nature of the boot loader in use, not something done on a regular basis.
Multibooters, Vista Dual and Multibooting - A Guide to the Multiboot Process
Updating the MBR and Bootsector - EasyBCD - NeoSmart Technologies Wiki