After being told by a few people in game that you have cannot use rending weapons on a noob, or that rending only goes up to level 5 vulns, I decided to do some research and supply a rending chart. Here is what I came up with:

MISSLE WEAPON DAMAGE
Skill 136 = Level 1 Vuln (x1.10)
Skill 160 = Level 2 Vuln (x1.25)
Skill 200 = Level 3 Vuln (x1.50)
Skill 240 = Level 4 Vuln (x1.75)
Skill 280 = Level 5 Vuln (x2.00)
Skill 360 = Level 6 Vuln (x2.50)

MELEE WEAPON DAMAGE
Skill 176 = Level 1 Vuln (x1.10)
Skill 200 = Level 2 Vuln (x1.25)
Skill 240 = Level 3 Vuln (x1.50)
Skill 280 = Level 4 Vuln (x1.75)
Skill 320 = Level 5 Vuln (x2.00)
Skill 400 = Level 6 Vuln (x2.50)

WAR MAGIC DAMAGE
Skill 176 = Level 1 Vuln (x1.10)
Skill 200 = Level 2 Vuln (x1.25)
Skill 240 = Level 3 Vuln (x1.50)
Skill 280 = Level 4 Vuln (x1.75)
Skill 360 = Level 5 Vuln (x2.00)
Skill 400 = Level 6 Vuln (x2.50)

Note: The rending/vuln system works on a SLIDING SCALE - you do not have to reach each threshold to have your rend land harder. It also looks like a buffed level 1 bow character would not see any great usage from rending weapons, but a level 15-20 bow character would be rending at a level 3-4 vuln level (Good enough for powerleveling).