Here is a link to the CDC's page that explains the difference between DTaP, Tdap, and Td vaccines: http://www.cdc.gov/vaccines/vpd-vac/pertussis/default.htm
You can also see an explanation on Wikipedia here: http://en.wikipedia.org/wiki/DPT_vaccine#Combination_vaccines_with_acellular_pertussis
I'm not sure if this will help you with your mapping problem or not, but hopefully it will. Also, I don't know how up to date the UMLS mappings are, but Tdap wasn't developed and approved for use until 2005 (DTaP was developed in the 1980's). It is possible that Tdap isn't in the UMLS system yet, or if it is, it may have been hastily added by people who didn't fully understand the difference.
In a nutshell, DTaP and Tdap are very similar, but the dosages of the three various components are different. DTaP has full dosages of everything and is typically given to children under age 7. It is usually given several times over a period of a few months. After that, booster shots use the Tdap vaccine, which has reduced dosages of everything except the Tetanus component (Tetanus dosage remains unchanged).