Yes, you can definitely achieve this in Excel using Conditional Formatting. Here are the steps to follow:

1. Select the range of cells where you want to apply the formatting.
2. Go to the Home tab on the ribbon and click on Conditional Formatting.
3. From the drop-down menu, select "New Rule".
4. In the "New Formatting Rule" dialog box, select "Use a formula to determine which cells to format".
5. In the formula bar, enter the formula =A1="V" (assuming A1 is the first cell in the selected range). This formula checks if the cell contains the letter "V".
6. Click on the "Format" button and choose the fill color you want to apply to the cell when the condition is met (i.e. when the cell contains "V").
7. Click "OK" to close the "Format Cells" dialog box.
8. Click "OK" again to close the "New Formatting Rule" dialog box.

Now, whenever you type the letter "V" in any cell within the selected range, the cell will automatically be filled with the color you chose in step 6.

To add another rule for the letter "S", simply repeat the above steps but use the formula =A1="S" instead. You can choose a different fill color for this rule.

That's it! Now you can easily track vacation and sick days in your Excel spreadsheet.