See page 13 of the
2i2 user guide. You want to monitor your vocal mic direct, not though Reaper.
If you want some reverb (from Reaper) in the vocal, you can use monitoring fx. Use a reverb that has no latency. Even if the reverb has some latency though, at least it won't be as bad as hearing your vocals "doubled".
Also FYI: ReaFIR will almost certainly add latency. Don't use it for a monitoring effect while recording. Look at the fx browser's bottom left corner to see the "CPU %" and "spls" (samples). If you see anything other than "0" spls when you have the effect selected, it is adding latency (the more samples, the more latency).