I have a very, very slow kitchen drain. I have replaced all the pvc piping between the sink and the drain stack in the wall and have rodded the main drain 20' down with a hand crank 1/4" rotary auger and it is clear, I suspect a clogged vent pipe.

Using the dishwasher it will back up into the sink, but then eventually drain down over a 45 minute wait.

I have acute fear of heights and cannot get on the roof but I do have a full attic and see the cast vent pipe above the kitchen and have access to about 3' of it.. Is it OK to take the sawzall and cut an access hole in the cast vent pipe for the flat steel rodding cable and clear the stack that way? Then I'd fab a thin sheet metal type patch over the hole secured with hose clamps?

This is in a Chicago bungalow about 70 years old.

Vent pipe is not the problem. A blocked blocked vent pipe will cause the trap to be siphoned dry, not cause slow drainage.

Don't cut that vent pipe. Snake the line again, if necessary snake with a power snake.

Thanks for the advice - I have tried rodding the line 4 times now out to the max of my hand held rotary auger - 20 feet - and don't meet up with a clog - I have a 40' flat steel auger of 40' but it won't go in the opening and make the 1st hard bend.

Other sources on the net suggest that a clogged vent pipe could present my symptoms so for the moment I remain confused.

I am working in a corner under a countertop between two cabinets so there is just not enough room for a rented power auger unfortunately.

Would it work to go into the basement and cut a hole in the pipe to fit the flat blade auger?

I note that this is the only drain with a clogging issue - the toilet, bathroom vanity and shower drain fine. But they are on a separate vent pipe.

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Other sources on the net suggest that a clogged vent pipe could present my symptoms

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Disagree.

Water flowing down the drain has to have air coming in behind it to take its place. If the vent pipe is blocked, the water in the pipe will pull the water out of the trap. Then air will come in through the trap and the water will flow continue down the pipe.
You can get some idea of how away the blockage is by how much water and how long it takes for the water to stop flowing.
A power snake would be best but you can probably do it with a manual snake. You said you have replaced the piping in the cabinet under the sink. If the cabinet is to small to work in, you can temporarily extend that piping to the outside of that cabinet with additional PVC pipe and snake from there .

What is probably happening is that the snake is going UP the vent pipe rather than down the drain. Use a snake with a drop head.

If you push that 40' snake in and do not find the blockage, go out side and look up at the vent. You will probably see the snake coming out the vent pipe.

Thanks for the suggestion - I will try and locate a drop head auger of at least 30 feet - I was pretty sure I was reaching down, just judging by all the wet black crapola that was on the spring, but I cannot of course be certain.

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wet black crapola that was on the spring

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Indication that you are going through a soft blockage and it is collapsing again after you pull the snake out ;
Or
You are just going through the accumulated sludge and have not yet reached the true blockage.

Keep trying. You will clear it eventually.

You need longer and bigger snake. Use 3/8 snake with spring tip. Run it at least 35' deep. Let me know if it is draining OK now... Back to you.. Milo

That, "wet black crapola" stuff is going to require the power snake as suggested by both guys above AND will also require you to flush the drain line with hot water using a garden hose a few times IN BETWEEN snaking the drain a few times. Here, snake the drain, and if possible, run hot water while you pull the snake back (cleans snake, too)... repeat this many times to end up with a long lasting job!

Mark