Stout gone bad?
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The forming of that pellicle indeed points to an infection. 
How long has that been in that "secondary" fermenter?
A large headspace is never good in secondaries. Oxygen can promote growth of unwanted organisms.
The beer level should be ideally 1-2" under the bung.
Now a slight sourness is not always bad or unwelcome. I've had some amazing tasting sour Stouts.
I'd say it's time to keg and keep cold to slow down the souring progress.
How long has that been in that "secondary" fermenter?
A large headspace is never good in secondaries. Oxygen can promote growth of unwanted organisms.
The beer level should be ideally 1-2" under the bung.
Now a slight sourness is not always bad or unwelcome. I've had some amazing tasting sour Stouts.
I'd say it's time to keg and keep cold to slow down the souring progress.
Basically there's nothing that can grow in beer that will make you sick. Low pH and the presence of alcohol are part of that protection.
The pellicle is not harmful, the (souring) bacteria* that cause it are already mixed throughout your beer. They try to close/seal up the surface to protect them from other organisms entering and intruding. It will grow in size and thickness with time.
* Brett could throw pellicles too, especially when exposed to oxygen. That could be welcome, but if the infection is unintentional, you never know what else came along with it.
Take a sample and taste it, and be prepared to package it soon.
No need to siphon off the top, you'd be wasting good beer. Just rack/transfer from underneath, even if some of it comes along it's fine.
But you do need to keg, bottling would be very unwise, unless you can pasteurize them, right after they're perfectly carbonated. You'd use champagne bottles for all security. But it's still a narrow edge.
So best off kegging that batch, keep it cold in your kegerator, and enjoy it early. It may sour more with time, depending on what got in there. You can transfer the wood and vanilla for some extra extraction over time. Just put a (mesh) filter or better a small mesh bag over the bottom of the long diptube, to prevent clogging it with pieces of wood and vanilla.
You should have "liquid purged" that carboy instead. Same for kegs, leave lid on and fill with beer through the liquid-out post. Look up how it's done.
Flushing a carboy (or keg) with tank CO2 is not very effective or efficient. But instead you could use fermentation CO2 (there's a huge volume produced in a 5 gallon batch) to take care of it for free. Same for kegs.
