Few tips, reminders and key points.
If your vintage leather bag gets stained by water or rain, the leather fibers got impregnated with water and the colour darkened. Remove and dry the excess of water with a towel, then use a hairdryer on the stain to help the moisture evaporate. If a dark stain remains, you can wax the rest of the bag (not the stained area) to make it look uniform again. The whole bag will look darker though.
If a zipper gets stiff, rub it with candle wax.
When you're about to use a conditionner, wax or cream on your bag, in doubt, test it on a hidden area of your bag and let dry to make sure it doesn't have an esthetic result that you dislike.
The leather needs to be nourished when it feels dry and rougher. The vintage leather will lighten. This can happen after 6 months to 1 year.
You don't need to treat and protect the bag when new. Your satchel has just been oiled or finished with a thin coating depending on the colour. It is ready for dust, rain etc. Leather is not "perfect". It will stain, get scratched...and eventually might look roughed up and unique. But regarding treatments, there is nothing to do on a new bag.
The vintage leather might stain your clothes. You can rub the bag with a clear cloth to check if your bag is susceptible to stain your clothes or not (the light coloured ones). In any case, this phenomenom doesn't last. A vintage leather will only cause stains on light clothing if the oils have not settled yet, it is a matter of days.
To preserve the threads and prevent tear. Don't drag the bag on the floor as the thread can be hooked onto a nail or sharp wooden edge for example. That is how a pocket can start tearing for instance.
Don't be disturbed by the shoulder strap fixation system. Sometimes, the strap can rub and lift the sides of the front flap. The flap won't tear away because of that. It can eventually wrinkle the leather, so will other parts of the bag. Discoloration, scratches, wrinkles, it's all part of a Ruitertassen leather bag's destiny. Live with it or buy another brand of bag.
You can rub most scratches away on the vintage leather. This leather is highly sensitive to friction and marks will appear easily on it, it is the result of the oils in the leather. You can buff this leather with a soft cloth and give it a uniform look if needed.