Marinate cleaned pork pieces with 1/2 tbsp vinegar and 1 tsp salt.
Soak kashmiri chillies, ginger & garlic, cloves, cinnamon, mustard and fenugreek seeds in 4 tbsp vinegar for 20 mins. Add 1 tbsp water to this and grind to an almost smooth (a bit coarse texture) paste.
Heat oil in a deep and wide pan. Add sliced onions and cook till it becomes golden brown. Add ground paste. Fry on low flame for 6-8 mins or till the raw smell goes and oil starts appearing. Add 1-2 tbsp water in between, to bring the masala together. Add marinated pork to this and mix well. Add 1.5 cups water and bring to boil. Reduce flame to lowest, cover and cook, for 30 mins. Open and stir in between.
Add chopped tomato (do a taste test and add tomato accordingly). Mix well. Cover and cook for another 20-25 mins on lowest flame, till the water is almost dried and pork becomes tender.
Do a taste test and add more salt and pepper, if required.
Notes
I strongly recommend you to slow cook the pork, instead of pressure cooking. The color of the dish changes as it rests. It will be bright red initially, turning darker red after sometime and later becomes almost blackish. If you prefer a really spicy vindaloo, increase the qty of dry kashmiri chilli to 25 and add 1/2 - 1 tsp pepper powder in the end. Make sure you open the dish and stir in between and also, add more water, if required. I used only 1.5 cups water. You can store this in fridge for 3-4 days.