I've been guilty of the same crime.
Myriama & I were once pulled over - well only Myriama actually despite us both being on the 'freeway' rather than in the 'bikes lane' - for being in the incorrect lane. It was on the way from HCMC to My Tho & I recall that despite covering the whole country it was only on the outskirts of HCMC that I saw the police operating to enforce this rule. We were only ever stopped on one other occasion - on the southern section of Eastern HCM Road (Hwy 15); this time by police looking for a payment. This - only twice stopped - despite over a year of constant long distance riding in Vietnam.
I agree the signage can be difficult but ignorance is no excuse & one should abide by the rules of the road. My point here, however, is to draw riders attention to what it's like on the 'bikes lane'!
This section of the road is only the width of a single lane & usually separated from the main road/highway by a usually cement barrier with a cement gutter on the other side. Such width means you are totally at the mercy of the slower (& I mean very slow) traffic. It is always CROWDED - remember there are over 16 million registered motorcycles in Hanoi; God only knows how many in HCMC!
The road surface is universally in a poorer condition, sometimes quite dangerous, to the (little used) main road. The heavy traffic in the lane makes it difficult to see dangerous conditions in advance & sees motorbikes directly in front of you swerving without notice.
Bikes enter the bikes lane from your right without caution - no-one so entering offers even a glance to see what traffic is moving by, the risk of a collision, or whatever.
In my case it was the safety issues as well as the fact that we were travelling long distances & looked to maintain a certain speed to arrive at our destination in a reasonable time that led us to flaunt the law on occasions. Unacceptable I know.
Stay Safe.
(Viethorse - did you or any of your friends make the paddle racing in Soc Trang?)