To do what you are attempting with proc tabulate I believe you would need to create additional class variables with the desired label for Var_a and Var_b and display the variables to suppress the labels. The new variables would obviously need to be on a class statement and the table statement would need to be modified to something like
table var_a_labeltext='' * var_a=''
Or move to proc report
April 27 – 30 | Gaylord Texan | Grapevine, Texas
Registration is open
Walk in ready to learn. Walk out ready to deliver. This is the data and AI conference you can't afford to miss. Register now and save with the early bird rate—just $795!