Format[OverVector[v_]] := Style[HoldForm[v], FontFamily -> "Arial Black"]

{q, r, OverVector[s], t, u, v}

vList = {}
defAsVector[x__Symbol] := (vList = Union[Join[vList, {x}]];)
defAsVector[] := vList
undefAsVector[x__Symbol] := (vList = Complement[vList, {x}];)
undefAsVector[] := (vList = {}; $PrePrint =.;)
$PrePrint = # /. {x_ /; MemberQ[vList, x] :> Style[x, Bold]} &;

MakeBoxes[x,StandardFom]:=StyleBox["x",FontWeight->Bold];
MakeExpression[StyleBox["x",FontWeight->Bold]]:=MakeExpression["x"].

declaredVectorQ[x_Symbol]:^=declaredVector[x]^=True;
MakeBoxes[x_Symbol?declaredVectorQ,StandardForm]:=
                                    StyleBox[ToString[x],FontWeight->Bold];
MakeExpression[StyleBox[str_String,FontWeight->Bold]:=ToExpression[str];

$PrePrint =.
x
$PrePrint = # /. {x -> Style[x, Bold, FontFamily -> "Helvetica"]} &;
x