Why do I get burning sensation in my stomach when I eat salt and vinegar chips?

Burning sensation. It sounds like pyrosis. As long as it happens with only one food, it's probably just a minor enteric idiosyncrasy. If it gets worse, I'd see a pyrosis specialist. Your other - and obvious - choice is to lay off the salt and vinegar chips.
Stomach pain. Stomach pain can be due to many causes, but what you describe with having stomach pain after meals can be due to ulcers, gastritis, gallbladder issues like stones, reflux, etc... See your primary physician.