I know you cant backtrack across either of the big ponds (Atlantic or Pacific), but, is it feasible to backtrack from Europe to Asia?

Im looking at flying MEX-FRA-PEK, then PEK-VIE (there would be the backtrack Asia-Europe, although I dont cross oceans by doing that), then VIE-SIN and continue back to MEX through either Japan or Australia returning through US West coast.

Im getting conflicting information here in Mexico City, travel agent says no, but LH said yes. Im inclined to believe LH, but would like to hear your comments/experiences. Thanks!!