I use all sorts of lengths, it depends what sort of portrait and where I'm taking it.
Close ups of the kids sleeping, it's normally 35mm 1.8.
Around out and about it's 55-200 (non VR) which IMO takes great pics for the price (it's not a fast lens at all). Dont buy this version though as the VR is something silly a fiver more these days. It's not good low light though, as always you can say most things are good - IF the light is good - that 'small' caveat
If I am honest with you, I could spend > £1000 and a 70-200 and see maybe a 5% increase in my 'satisfaction rate' from the 55-200 - that's me personally, it IS a different league, but I am trying to say it's all what you are happy with - a lot of people spend a lot of money for mediocre pics IMVO.
Low f number for OOF backgrounds and lower light.
Don't think there is anyone one lens for so called portraits tbh, all depends on subject and environment and ultimately what 'you' like (assuming someone isnt paying you

). All subjective. You might be in a studio (?) though and the answers will be slightly different - controlled lighting, easier working environments - adults following instructions as opposed to young kids/babies

...who knows (?)
Also, VR for me isn't very imprortant - I am reasonable at handheld at low shutters and my kids always move anyway, so it's no deal breaker for me either way. Think people get to carried away with all that stuff (end up with a technically good photo that is boring as hell - normally on a white background)
My next lens willl probably be a Nikkor 85mm or 105 (or if I never get round to saving a Tamron 90 or something like that) as I'm developing a background interest in Macro, and will also double this up to be quite a nice 'portrait' lens.....ie. just trying to think ahead a bit and 'compromise' with gear - it's all compromise.....