On vsphere set_mtu does nothing because bosh uses manual ip assignment. I'm not sure why this job doesn't warn people that it only works with dhcp.
What would you think about changing this to use udev rules? Here's a replacement template
#!/bin/bash
set -eu
INTERFACE=<%= p('networking.set_mtu.interface') %>
MTU=<%= p('networking.set_mtu.mtu') %>
UDEV_RULE=/etc/udev/rules.d/70-networking-set_mtu-$INTERFACE.rules
# Override MTU setting
cat <<EOF > ${UDEV_RULE}
SUBSYSTEM=="net", ATTR{name}=="${INTERFACE}",RUN+="/sbin/ip link set mtu ${MTU} dev '%k'"
EOF
# Enable new MTU setting now
/sbin/ip link set mtu $MTU dev $INTERFACE