renting a bicycle

Google it.
Just looked thru the Lonely Planet and Tripadvisor posts and they give you internet sites to visit......also point you to the backpacker area where bicycles can be hired
Another is Bikesbooking.com
The posts do say however that some bikes are not in very good condition,other posts say it's cheaper to buy a bike...depends how long you want the bike for
You will be able to buy a lock anywhere...lots of shops selling them...little more difficult in the city center

Consider reading this thread.  https://www.expat.com/forum/viewtopic.php?id=637996  The OP bought her used bicycle for 1.5 m VND or roughly $65 US.  She bought a mountain bike but you could probably find a one speed for even less, say $50, ride it for a few weeks and sell it back to the same place for $20.  Net cost $30.  Maybe better than looking for rentals, haggling over deposits and all the rest.  You could even arrange to give it to the doorman at your D1 hotel when you leave and he will take extra care to watch it for you when you are not using it.

I know a place where you can rent a bike (only 100k VND/day - 4$ US/day) with a wide range of choices (from city to off-road bike). I often come here to get a bike for a weekend short trip. However, I could not remember exactly the address (on Đinh Tiên Hoàng Street - near Điện Biên Phủ roundabout, just 2km to city downtown). The only concern is that they require to keep any of your identity document (passport, ID, Driver license...). So if you dont feel bothered by this, I am pleased to take you there.

Thank you , i'll look into it , so they may hold my drivers license instead of my passport?
that sounds pretty good.

They need to keep any of your identity for "deposit", in case you dont prefer this way, you will need to place a deposit for the bike.

I just found they have a website with contact details as below:

http://xedapvietnam.com.vn/san-pham/cho-thue-xe-dap-268

100.000d - man, you can rent an automatic motorbike for 120.000d at Chi's.

That's great info , thank you very much for all of your research . i'll email them.

100k a day for a bike thats worth about 1 million vnd, yeah right.