I'm indeed using 5.4.14074 on both computers.
Marnix's suggestion was instructive. Here's what happens :
Running a test instance of VEP with only two VIP channels, I got the feature to work on the slave, too.
So I came back to my template and tried again. My template has VEP running three instances : one for winds and percussion (VIP only), a second one for strings and percussion (a mix of VSL and Kontakt) and a third one loaded with various EW Play instruments.
The MIDI Activity Focus works fine on the VIP-only instance, but not on the other two. On the mixed-format instance, the feature doesn't work even when triggering events on a VIP channel while viewing another VIP channel.
So I'm wondering whether VEP "doesn't like" having none-VSL instruments loaded (which would be weird). Could this be a bug?