Previous analytic models of TCP Vegas throughput have been developed for loss-free (all-Vegas) networks. This work develops a simple and accurate analytic model for the throughput of a TCP Vegas bulk transfer in the presence of packet loss, as a function of average round trip time, minimum round trip time, and loss rate for the transfer. Similar models have previously been developed for TCP Reno. However, several aspects of TCP Vegas need to be treated differently than their counterparts in Reno. The proposed model captures the key innovative mechanisms that Vegas employs during slow start, congestion avoidance, and congestion recovery. The results include (1) a simple, validated model of TCP Vegas throughput that can be used for equation-based rate control of other flows such as UDP streams, (2) a simple formula to determine, from the measured packet loss rate, whether the network buffers are overcommitted and thus the TCP Vegas flow cannot reach the specified target lower threshold on throughput, (3) new insights into the design and performance of TCP Vegas, and (4) comparisons between TCP Vegas and TCP Reno including new insights regarding incremental deployment of TCP Vegas.