Hi,
The convention is usually the first letter is the most positive. Vab would mean the voltage across a and b where a is the most positive. An arrow that 'looks' like a vector is sometimes used to show the voltage where the tip is the positive. The arrow would then be marked "Vab". So in your drawing this arrow would be pointing east to west, the tip is 'a' and the tail is 'b'. The current arrow points in the opposite direction.
As for current, two nodes are not necessary so it really should be Ir where r is the object that has I amps through it. If you need to show direction then you would still follow the first letter most positive convention. That means Iab would be conventional current flow from a (most positive) to b (most negative), where again the most positive is in reference to the voltage not the current even though it is a current being specified.
I almost forgot to mention, that sometimes the polarity is already shown using plus and minus signs, and the voltage is then given as Vab or Vba, which mean opposite polarities. For example, if the plus sign is at 'a', then Vab means a positive voltage, and Vba means that same voltage but with sign reversed. So say we have a 10 ohm resistor drawn horizontally, with the left side with a plus sign and the letter 'a' and the right side with a minus sign and the letter 'b', and we have 1 amp flowing through it. Since the left side already has a plus sign, that means the left side is more positive than the right side, so Vab would be Vab=+10v, and Vba=-10v.
If we were to draw an arrow, we would probably draw it from east to west where the tip is positive, then we can label that arrow as Vab or as +10v, but it is possible to draw the arrow from west to east and label it "-10v".
So the rule then is that polarity signs (plus or minus) have the highest precedence for showing the polarity, with arrows being next if there are no plus or minus signs. Actual voltage levels referenced to ground or to the object are good too.