Thanks a lot Karsten!
This helped me! I tried always "v Ctrl+_ i", didn't have the Idea, that "v" and "i" have to be two different Strings!
You can read Blanck(_) for more information about the meaning of _ in Mma.
You should use
given = {"d" -> Quantity[100, "Meters"], "t" -> Quantity[10, "Seconds"], Subscript["v", "i"] -> Quantity[5, "Meters"/"Seconds"]}
or type "v" Ctrl+_ "i" as the short form
"v"
"i"
