Would it be worthwhile to buy Euro Train Pass just for these two legs of traveling?
What does "worthwhile" mean? It would certainly be much more expensive to buy a EURAIL pass. With advance purchase from Austrian Rail (www.oebb.at), you can get tickets for Vienna to Prague as low as €29/p. Similarly, with advance purchase from the German Rail (Bahn) website, you can get tickets from Prague to Frankfurt for as low as €39/p. These are date and train specific tickets with limited refundability, but considering the savings, worth the commitment.
Thanks for your feedback! Good and practical info.