vncpasswd is used both by vncviewer and vncserver so it should be always unconditionally installed, currently it's only available with the USE server.
vncviewer doesn't use it directly but the user might use -PasswordFile to specify a file generated by using vncpasswd
Thank you for your report. This commit fixed it. https://github.com/gentoo/gentoo/commit/69bbdfc9b133a5b144d007289399d2db1008f672
Reopening, from what I have seen in the git repo, only version 1.7.1-r2 has been fixed, the other two ebuilds are not fixed yet
<1.8.0 ebuilds removed from repository.