Hamzah - on each subsequent iteration of your loop, you are overwriting the data from the previous iteration since you are setting vt to something new. You need to concatenate the new data with the previous data so that you don't lose anything. Try the following:
x = [1,4,-1,5,0,2];
vt = [];
for k=2:length(x)
if x(k) > x(k-1)
vt = [vt x(k-1):0.1:x(k)];
else
vt = [vt x(k-1):-0.1:x(k)];
end
end
