Is there a 64bit version of EngageOne Vault?

No.  Currently all versions of Vault are 32 bit only.  That means that any supported 64 bit Operating System will need to be able to have the ability to run 32 bit applications.  Per the EngageOne Vault 7.6 Installation Guide:

Linux distribution (32-bit or 64-bit x86) with kernel version greater than or equal to 2.6.18, and glibc
version greater than or equal to 2.3.
Note: Running on a 64-bit Linux version will require the installation of 32-bit compatibility

Vault and Render servers are supported on the following:
• Microsoft Windows Server 2008 R2 Service Pack 1
• Microsoft Windows Server 2008 Service Pack 2
• Microsoft Windows Server 2012 and Windows Server 2012 R2
• Microsoft Windows Server and Datacenter 2016 versions that support 32 bit applications.
Note: that the Windows 2016 Nano Server does not support 32 bit applications.
The following is supported:
• 32-bit x86
• 64-bit x64 (runs as a 32-bit process)
It is highly recommended that you use a 64-bit operating system to host Vault. Vault servers can
put considerable stress on the kernel resources of 32-bit versions of Microsoft Windows.

As of this writing there are no plans to create a true 64bit version of EngageOne Vault.
UPDATED:  June 22, 2020