How long have you been having those lunches or how many occurrences were there, up until this point? Because the way you said it, it sounds like many of them, so I'll assume so.
Vegetarian restrictions do make things a little bit more complicated, but restaurants for vegetarians or vegans have been on the uprise for the past few years, so it is very possible that you'll find one on your area. If you state where you are it would help as well.
Personally whenever I'm involved in this kind of conflict of different choices for going to lunch or dinner, I try to find a compromise of going into something that will satisfy both. Indian food tends to be a bit spicy or with a lot more seasonings than some cultures are accustomed too, I don't know if that's the case or if you just don't like it.
One thing that can work is just stating that you always go to the same type of restaurant and that it might be fun if you try a new one. If you think they'll think that "Michael just wants to go to the place he loves" you can try to counteract with "I found out about these places (2, 3 or a handful) which I never went, but are well reviewed/spoken about. Can't we try it some time?"
Bottom line, whether you like it or not, you can just argue that varying would do no harm! You might even want to vouch for a restaurant of your culture or any of your non-Indian colleagues by saying "it'd be great if you could try our cuisine" :)
They might not be inclined to change it, because they're accustomed to going to their preferred cuisine, so you could:
Try finding a place that has their cuisine + another one.
- Ask for something new there, which you think they'll like
- Vouch for them to try/taste it
- Hope they like it
Order food or cook something on any other day and have them try.
Both approaches try to work with the approach of making them try something new that they might like. Number 1 worked when I went to a place with a friend and he always ordered the same things. I picked something and told him to try and he genuinely admitted it was better than the things he'd been having and thanked me.
You don't need to expect such a positive response all the time, but if they at least like it or are willing to try it, you can propose a new place next time, or maybe some months from now.