The vendor name or bank account name disappears from the list when you create and save a bill, bill payment or check by right clicking the vendor name in the vendor list or bank account in the chart of accounts while the Vendor Center is open. This also applies to modifications, deletion of these transactions or editing the vendor's profile.
There are two options you can use to make the vendor appear in the list:
Workaround 1 - Resort the lists
Workaround 2 - Change filters
Note: Selecting this option will also show all vendors marked inactive.