Villa and Dortmund are underdogs so going through to the SF is going to be hard but not impossible. Porto were underdogs in 2004 but won the CL because Jose Mourinho had a talented team. They won the Primeira Liga, Taça de Portugal and UEFA Cup in 2003. Villa and Dortmund are not as talented as Porto were in 2004 but they have a slim chance against PSG and Barca.
Aston Villa and Dortmund are two underdogs in quarter final phase of Champions League but recall some underdogs like Monaco, Porto, and even Dortmund in some seasons, they managed to go to final matches as underdogs for several times. They are weaker than themselves some seasons ago but chance for making another surprising season in Champions League is not impossible for Dortmund.
When you are talking of underdogs in this champions league you are not suppose to mention Aston villa because they are not and Dortmund on the other hand are not underdogs because two or three seasons ago Dortmund was so strong that they qualified for finals though this season they are not in a good form and even if they were, they can not beat Barcelona. Aston villa beat Bayern Munich at the group stage and now they are at the quarter finals and i am not convinced that PSG will knock them out.