Yep - malt vinegar is just right on steak fries. Skinny ones demand ketchup. Or fry sauce.
White vinegar. With ketchup. And salt. My dad’s American, and my mom’s Canadian, so this kind of works out.
And for Chronos. . .all I’m looking for is the vinegar tang. Any other things sort of change the flavor of the fries as well as adding the tang, and I don’t like that. Using white vinegar makes them taste more like salt and vinegar chips, which is awesome.
“Fry sauce”?
The fries end up soggy and whatever flavor enhancement there may be is completely outweighed by the mush factor.
But if other people want to do odd things to their food, who cares?
I’ve never tried it, actually, but the smell is so strong that I’m not sure I could get it past my nose. :o
I once agreed with your grandfather.
I’ve seen what the French to do fries as well. Sacrilege.
Then the kids at the office dragged me to 5 Guys Burgers and Fries.
They talked me into trying the damned vinegar on fries. It’s actually quite good, and I’m man enough to admit it.
Now, get off my lawn.
Mentioned upthread. Mayo+ketchup. Evil. edit: Actually, I kid. It’s not that bad, but I prefer either plain ketchup or plain mayo if I’m going to go that route. Or Tabasco. It’s like spicy vinegar!
Saved on the edit. Otherwise you’d never be welcome in Utah!
Want relief for a stuffy nose?
- Prepare fries piping hot.
- Douse fries in white vinegar and salt.
- Place nose to within two inches of fries.
- Inhale deeply.
Clears a stuffy nose right up. And is a tasty treat, besides! 
The thought of the smell of salty and vinegary hot out of the oil french fries is making my mouth water (and I just had supper).
The first time I ran across vinegar for fries was in Thunder Bay, Ontario. I was 11 and with my family and the waitress gives me my french fries and says “Would you like vinegar with that?”
I was like “Why would I want vinegar”? Then she told me that’s how Canadians eat it. So I tried it, yeah that didn’t work out well. I’ll went back to ketchup.
I used to love getting High Street fries in Eugene, Oregon. Big basket of freshly-cut fries - sprinkle malt vinegar on them, add some hot sauce, and then dip them into ranch.
Yum!!
Most who are saying this are pure-blood Americans, like I am. 
Ranch dressing. Or rand chressing mixed with hot sauce. It’s especially good with cheese fries.
Five Guys fries + malt vinegar = awesome
Malt vinegar, of course.
The way to avoid the mushiness mentioned by several posters is just to put the vinegar on the chips you’re about to eat, not the whole plate at once. Eat those, then sprinkle some on your next target area, and so on.
Mrs Piper and I have tomorrow off and we’re going to our local pub, Beer Brothers, to treat ourselves to lunch. There will be vinegar. And chips. And mussels. And, just perhaps, a smidgen of beer… (last time we were there, Mrs Piper treated me to a top-end Innis & Gunn Scotch Cask Ale. 
)
Hmmm … you know I can be in your city in six hours, right? 
Aw, darn, just remembered. Tomorow’s Friday; I gotta work. 
Another time. 
It must have been about 1984 or so. Bitterly cold day. I was at the Lemon Plaice in the Bicester high street. Walked in, ordered a banger and chips. Doused said banger and chips with Sarson’s Malt Vinegar until the paper was soggy.
Best chips I ever ate. It’s got to be malt vinegar too. I’ll do ketchup on fries every now and then, but when I’m in the UK I go nuts at chip shops.
This thread reminds me, someyear ago, I went to a computer conference in Salt Lake City, Utah.
While there, I went for lunch to what I believe was a Denny’s, I ordered a Club Sandwitch with fries. When the waitress brought it to my table and asked if I needed anything else, I asked as I normally do here in Canada, for some vinegar.
She said, “Vinegar”. And I anwered, “Yes, please”.
So she walked away and came by a little later … with some vinegar in a little saucer bowl.
So, I simply use my teaspoon and sprinkled some on my fries.
At some point, I looked around the place and noticed the same waitress and another one watching me, probably wondering what the hell I wanted some vinegar for, because, from where they stood, I do not think they could see me putting some on my fries.
In my experiences in Florida, most places did not have it on the tables but could at least provide it in the appropriate dispenser if asked.
Malt vinegar and salt on chips / fries is very nice, but I prefer tartare sauce or garlic mayo or ketchup.