Vit B Complex / BBT Thermometer questions ??

Hannah
I'm on cycle 3 TTC and have noticed that my LP is on the shorter end. O'ing on CD 18-19 on a 28 day cycle. I read several places that vitamin b complex will help lengthen LP, however now it seems that I'm not o'ing at all! I'm on CD 21 with no temp rise. Anyone else with similar experiences? 
​Also I've noticed that every single day my bbt is either 97.7 or 97.5. My thermometer will go higher (had hubby test it) but is it normal to get such regular temps like that? This is the first cycle where they're so regular... Could this be vit b too?!