In VectorBus class, there's an implementation of flush_tx_buffer relying on xlCanFlushTransmitQueue function of the XL driver. However, this function is a no-op for devices other than XL family, which are discontinued. This means that the library does not support flushing the TX buffer for any Vector devices currently available on the market.
(https://cdn.vector.com/cms/content/products/XL_Driver_Library/Docs/XL_Driver_Library_Manual_EN.pdf)

My implementation of flush_tx_buffer consists of “sending” a dummy message with overrun/highprio flag set. The Vector interface will flush the TX queue after receiving a request to send this message.

Tested on VN5610A, for both FD and non-FD bus. I think it's a good idea to check this on other Vector devices, especially on XL family, to make sure that the implementation works for them all. In our use case, the flush is called after XL_ERR_QUEUE_IS_FULL is raised (e.g. when sending a ton of messages on a dead bus). In previous implementation, any next send request will raise this error if the bus is still dead, because the flush was a no-op. In my implementation, next send requests will not raise an error (until it fills up again).