I have heard about the IPSec VTI problem at VyOS meeting in Japan.
Mr. Asama said that this problem might solve with this patch.
http://bugzilla.vyos.net/show_bug.cgi?id=183
But currently, this patch was reverted.
http://bugzilla.vyos.net/show_bug.cgi?id=183#c3
It seems that this problem occurs when another vendor IPSec client (IE Yamaha RTX Series) connect to VyOS IPSec server.
We'll update about the problem later.