Found your reply while I was looking to book my tickets!
I'll take your suggestion and connect in Gia Lam. This gives me a solid 1:25 connection time - I'll take the train arriving Lao Cai at 06:20 so I can catch the sunrise going in.
Caveat is that I can only book with VN railways rather than the private outfits. - At least, that's what baolau.vn shows. (That's the site recommended by Seat61.)
380k VND for a soft sleeper - Baolau doesn't allow me to specifically reserve an upper berth. Is that always the case or is there a way to reserve berths, before they all get taken by passengers boarding from the start? How is soft sleeper?
In brief - yes, you should have enough time, but there might be a better option.
Firstly - if you've not booked your tickets yet, you could book from Haiphong to Gia Lam, and then from Gia Lam to Lao Cai. Gia Lam is a junction station on the outskirts of Hanoi, and both of your trains stop there about 20 minutes before/after Hanoi main station. My concern if you've already booked tickets from Hanoi would be that they'll resell your bed if you're not in it (or that bureaucratic Vietnamese train rules don't allow starting short), but you should be able to ask, and potentially get tickets changed, at the tourist counter at Hanoi station if you're going that way first (to the right of the main ticket hall).
This is unlikely to work if you're taking one of the private sleeper cars to Lao Cai, as you'll need to pick up tickets in or near Hanoi station.
If you do go to Hanoi: If they make you go all the way around, then it's about a 10 minute walk with bags (turn left out the main station, after a few hundred metres first left onto Nguyen Khuyen and over the railway, first left again onto Tran Quy Cap). So even if you need to pick up tickets for your Lao Cai train, you'll have plenty of time even if your Haiphong train is a little late. If you're near the back of the Haiphong train though, you might as well see if they'll let you across the tracks straight to the other station.
If your train is a bit late in, it'll also block the departure (there's about 15 minutes of single track out of Hanoi as far as Gia Lam) of your train to Lao Cai, and the departure of the train before your train, so everything should be pushed back.
Finally, if things do look to be running desperately late out of Haiphong, you could also see if a taxi would take you to Hanoi. I'd guess that on the meter it'll cost about 1.5 million VND; 70 USD or so. There's a new expressway, so it should take less than 2 hours.