I did the LT end-2-end NoBo in September 2013, finished at Canadian border on Sept 22nd.
used a Montbell Torrent Flier goretex jacket and it was the perfect system. I was able to get this on sale for half the current price.
https://www.montbell.us/products/disp.php?p_id=2328280
Has pit zips to vent when you get warm on uphill climbs.
what others have said here about Sept weather in VT is absolutely true. There were several days with rain, sleet and temps in low to mid 30s and 40s.
then there was the say I took a zero in Rultand when it was 90* and humid, just before that f'n cold front came thru. Sept weather is crazy up there.
don't forget your hands either. I started at Williamstown, MA with a pair of Nike thin running gloves, worked great til I hit Lincoln Gap. Going over Mt Abe it was low 30s and drizzling rain, and my hands turned numb from the wet/cold. Stopped in Waitsfield at a gear store and bought some better Outdoor Research gloves and need them the rest of the hike.
I would not bother with a poncho myself because of the wind on the ridgelines, a poncho would be more of a hurricane flag in high wind and rain.
but everybody has their own preferences, have a great hike