The Ubuntu plasma-vault
package page for 18.04/bionic doesn't mention "ecryptfs" in any of the dependencies, so I'd say no.
But, there's nothing like getting a custom answer about your specific system, so you can always simulate a remove (or install) to see what other packages would be removed (or added) with a package manager interface like Synaptic, or just
apt-get --simulate remove plasma-vault
The relevant lines should be similar to this non-vaults example:
The following packages were automatically installed and are no longer required:
libecryptfs1 libtspi1
Use 'apt autoremove' to remove them.
The following packages will be REMOVED:
ecryptfs-utils
--simulate
option in a terminal and see what else removing the two ecryptfs pacakges would want to do - just don't actually do it. Synaptic shows changes before actually applying them – Xen2050 Feb 19 '19 at 02:12