The "eng1" 20 amp fuse blows and van stops.....replace fuse, van runs fine........suggested fixes for this

then try to put 30 amp fuses it will be fine

Insure that the 20 amp fuse is the correct size. Might it be shown to use a 30? Otherwise, when compressor was replaced, was drier, orifice tube replaced and proper amount of oil added?

Open the hood driver side you will find a black box open it. There you will find the fuses but may I suggest that you remove the battery cables first . If you get a spark you could blow the other fuses while removing the fuses.

Keep installing a larger fuse & you will
someday burn a car to the ground
If a fuse blows once there is a reason
Replacing it one time seldom makes for a fix

engine light stays on
the ecu will have fault codes stored to be read
that will point to the cause and a fix of the over load

my best information on a vito if diesel..says
10 amp fuse 41 . glow plugs


you must need the van on the road very badly
30 amp in a 10 amp fuse position can
melt the wiring even a fire

I suggest a repair shop that can test the O2 Sensors
& check out all your other problems

Fuses only blow if-Old,Overheated or you have a Short circuit.
That should be any easy fix,to the extent that there aren't many 40 Amp Fuses and you will need a wiring diagram to follow the problem from the fuse to the cause. If you know what fuse it is, you can guess,but I wouldn't recommend that,it will be time consuming and maybe costly ,if you don't have a test procedure to follow and throw parts at it.
Don't replace the fuse.Their expensive and will keep blowing.

make sure of two things one is any wires touching at the starter to ground or melted and or is the starter drawing too much amperage causing the fuse to blow.starter may have a dead short in it.

Found a corroded / shorted wiring harness below the TCM in front of the left front wheel.  Repaired the wires - now works fine. 

You have a shorted engine harness, this is not common but it does happen, heat and engine movement cause damage to the harness, this is most common on hi miler cars and vans, the only way to fix it is to start by looking anywhere the harness rubs, that is where the damage occurs.