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"
