2.5 TB isn't very much in a month, especially not for a proxy of a much bigger site. Vultr's bandwidth is notoriously tight for the price, too. I feel like this is the wrong service for that type of host.
Cloud pricing for bandwidth is really limiting for a lot of applications. For comparison you can get a 1gbit/s line with your server on OVH EU for about $100/mo, which you could utilize for a theoretical ~321TiB/mo total transferred
nitter.net is an official Nitter instance (and most overloaded) known to be rate limited. But report to devs to check if there is other case or bug.[0]
BuyVM offers a plan comparable to their 12 dollar one for 7 dollars, and you get at least 16 TB of monthly bandwidth. I say 'at least' because they do not cut you off, and throttling is manually done and a last resort.