They're a Cycas species, but don't know which one. Not unusual for them to get gnarly trunks like that, and the branching as well. There's a lot of different species but a good possibility the one in the photo is a local Vietnamese one. Below is a good web site on them, you could check through it for a match.

Thanks tropicbreezent
Welcome the comments which confirmed most of what I suspected about them. Sorry to say the link didn't come through. :-((

Fof

That's strange, the link was in there at the bottom of the post before I pressed submit. But I didn't look again after. Anyway, this is the URL:
http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/cyckey.html

Thanks tropicbreezent. It came through this time.

Fof

That is some link.
I don't know if it comprehensively covers every species of Cycas, but based on just a) size & b) inferred locale, it can only be C. pectinatus. C. elongata would have been another option but its range appears to extend N to a point sum 1000km S of Hanoi.
Once I manage to translate the various botanical descriptors :-)) (my field is electronics) I shall see if I can pin down any other confirmatory details.
Thanks again.

Whats the round thing? A solar dryer?

stanaofh

No. it is one of those totally uncomfortable egg chairs. You can see a hanging one on the right.

I think just because these are in Hanoi, that they are not necessarily native to that area... Cycas circinalis/rumphiana have been the trade for many many decades and these could easily be a common species like that, purchased locally and grown for a long time.

In areas where Cycas species grow naturally it's common for people to bring seeds home and grow them. Seeds are highly visible/prominent so attract attention, and they are easy to germinate.

palmbob
I hear what you are saying, but taking into consideration their size - 4m and the slow growth rate of Cycas spp, - I would be prepared to state that these are old plants. The spp you suggest, C. circinalis is only found native in India, while Cycas rumphiana is an invalid name. Sure, it is theoretically possible to be just about any cycad spp if you say that "their seed have been traded for many decades."
I believe that the possibility is quite high that it is C. pectinata, based on size and therefore probable age. In addition it appears to be the only native spp that attains that height, that is found in the area, and is not uncommon. Also there are very lax environmental laws in Vietnam and, I hate to have to admit it, but money very commonly "greases wheels" in the country.
I agree with tropicalbreezent's suggestion for a lot of countries but not for northern VN. Land there is very highly prized and is needed for cultivation. I have been going to Hanoi for the past 26yrs and this trip is the only time I have seen any gardens of any sort, and these were in a very exclusive estate catering for foreign diplomats, business men and the uber rich Vietnamese.
Houses in Vietnam are built using the smallest footprint possible - 1 room wide, 2 deep with often 4 or 5 floors. The only places to dry clothes or grow plants are on balconies.
The thought of planting and growing cycads for future growth just would never be considered, especially as wild plants can be easily collected.