You are right,
There are some differences, between VoIP and VoMPLS.
1a.- VoIP is voice over IP packet, where the Payload is encapsulated with its RTP/UDP Overhead on IP PAckets.
1B.- VoMPLS is Voice Over MPLS Packet, wher the payload is encapsulated on a packet with MPLS Overhead (Not RTP/UDP Overhead), because is not and IP Packet.
2a.- Voip, could be use din every environment, LAN, WAN, PC, MG, RGW, an soft application.
2b.- VoMPLS, is only to trasport voice stream over a Pure MPLS Network for Trunking Application basically. to do that the new generation of media gateway will do two fuctions, the first one, to code the Analog Voice and convert it to a digital stram, and the second fuction will be a LER (Because each voice stream, will be encapsulated in a packet in a corresponding LSP).
3a.- With Voip using a G.723.1 coded, you will need a least 17 Kbps of bandwidth for one channel
3b.- With VoMPLS you will neen only 8,5 Kbps, due a redcution of IP Overhead.
There aree more differeces but this space is to small to explain it