Well, one thing to keep in mind is that on intercontinental flights round-trip tickets still tend to be significantly cheaper than two one-way tickets. It may be a better idea to book a roundtrip into RIX or into Germany.

If you're dead-set on two one-ways, consider flying Air Berlin back to the States from Germany - they are one of the few European airlines that offer one-way pricing even on their long hauls. Oh, and Condor does so as well, and often has reasonable deals on their nonstops to Vegas from Frankfurt.