Traceroute to Virtual Server Loops between VIP address and BIGIP's Default gateway

not totally sure but the difference between normal traceroute and traceroute -I is that you use ICMP echo with the extra flag and UDP packets by default. it is possible that one of your virtual servers picks up the UDP traffic and sends it somewhere unexpected. the ICMP traffic is left alone.

do a packet capure with -i interface:nnnn and it should show you which VIP is involved.