My plumber says code will not allow an elbow for the vent pipe until the pipe is at lease 6" above the level of the sink. Any idea for the reason behind this rule?

I think your plumber was referring to "reventing". That's when a fixture on the first floor Connects back into another fixtures vent that goes through the roof. This revent connection MUST be connected at least 6" above the flood rim of the second floor bfixture. A dry vent can have as many bends as you wish as long as it slopes back to the fixture it's venting. Hope that makes things a little clearer. Now you got me curious, As a rule a kitchen sink drain runs horizontal for 5' into a tee that vents directly though the roof. What makes yours different? Cheers, Tom

In my area you cannot not offset any vent 90 degrees until you are 6" above the flood level of the fixture so it will overflow the fixture before it backs up horizontally onto the vent