Cisco WAN :: 4506 / EIGRP Neighbor Not On Common Subnet / Packet Loss

I was called into look at a customers's site that was upgraded by another vendor. They were complaining about packet loss and performance issues. I discovered they had changed from static routing and turned on EIGRP (100) and the EIGRP neighbors are not on a common subnet, they used secondary IP addressing to connect two 4506 switches to the core router. Correct me if I'm wrong, but won't this cause the neighbors to bounce and cause the degraded network performance? Don't we need to have a single common subnet for this to work? Either that or they have bridged the VLANs somehow? There is also a cable connecting the 4506 switches besides the uplink to the core router.
 
4506#1
 
May 17 21:28:55.443: EIGRP-IPv4(100): Neighbor 192.168.120.1 not on common subnet for Vlan1
May 17 21:29:05.848: EIGRP-IPv4(100): Neighbor 10.102.10.10 not on common subnet for Vlan20
May 17 21:29:17.327: EIGRP-IPv4(100): Neighbor 206.78.xxx.xx not on common subnet for Vlan20
May 17 21:29:48.291: EIGRP-IPv4(100): Neighbor 206.78.xxx.x not on common subnet for Vlan30

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