HSEROUT of array of data with termination on a null

[Dave]

That's great but, at 9600 baud it will be in this routine for 26.5 milliseconds. That's OK if you can afford it...

[tumbleweed]

...trying to offload up to 256 MB of data from a flash chip to VB on laptop over 115.2k BAUD connection.

At 115K it takes ~22ms to send 256 bytes assuming back to back transfers. At 9600 it's over 10x longer than that.

You could use background serial transfers while you do something to read that flash chip, but you're going to have to know what you're doing to get all that working.

[amgen]

you could...... forget hserout, then you can make your data buffer bigger, use longs if your pic can do it.... 18f's, then write data to ram or program mem location that can store more than 256 bytes, then access data knowing starting location and loop through data with TX routine number of chars to send. I have sent 10K bytes at a time serially at 115K over 8266esp
I would dig up some code if interested in that technique.