I've almost always gauged my "emptiness" by miles. I drive Syncros, so
my gas mileage is in the 18mpg range normally. Since I have a 18.5
gallon tank (which I try to make sure I fill up entirely when I refuel)
this means I should be able to travel  333 miles to a bone dry tank. Of
course that would be fuelish.

So to give myself some cushion, I usually fill up when I'm down to 4 or
less gallons. That still is about 270 miles on the old trip meter.

Now my gas gauges frequently are well into the RED section by that
point. Or sometimes even pegged at the bottom.

If I'm doing 4wheelin', my mpgs will be less (as they will if I've been
doing a lot of towing or AC usage.) But even at 16 mpg, I should be
able to get 296 on a tank, so 250 miles is still pretty safe.

Now 170 miles on a tank is very short (granted that the 2wd Vanagons
have a smaller tank...they also get better mpgs.) You are running rich,
have a fuel leak, or a combination of both. Do you smell gas after your
engine has been running for a short amount of time? That could indicate
a fuel leak.

As has been mentioned frequently, check and replace those fuel lines.
The minimum age on anybody's vanagon is 12 years, and yours is 18 years
old. Rubber does deteriorate over time, and fuel lines are no
exception.