we are going to Hue, Danang, Hoi An and HCM City and staying in the very best hotels in these places and will eat at the top French restaurants. Should I take a suit and tie or would everywhere be very informal and I could get by with longs and a t-shirt

No need for Suit and tie. Sure, you will see people in suits in places like the Metropole; but you will be perfectly comfortable in more casual dress. Keep in mind that Ha Noi can get quite cool in winter, you might be a little under-dressed in a Tee for weather reasons.

I wouldn't pitch up at the top hotels and restaurants wearing jeans and a tee shirt. You'd probably get in okay but would feel decidedly out of place. Slacks and a casual shirt or polo shirt would be acceptable with shoes rather than trainers. Other than that it is a very informal and casual country.

Da Nang and Hoi An are very casual indeed and I have trouble thinking of a place I'd describe as "top", HCMC has it's hot spots but I've even been to the rooftop bar in the Rex in shirt and jeans and nobody even raised an eyebrow.

I was there in 1970/71 and the dress code was green baggy

I was there in 1970/71 and the dress code was green baggy

Popular in Hanoi until the 90's; not so much these days.

I wouldn't pitch up at the top hotels and restaurants wearing jeans and a tee shirt

Had more than the odd beer at the Metropole Pub wearing Jeans, nary an eyebrow raised. And certainly not a problem at the Hilton Sports Bar.

Thanks everyone for the info' I hate wearing a jacket