I'm running one thread for the purposes of debugging, this will eventually have to be set to ~8-16. The logic all works and based on my reading of the documentation the reason I'm not getting anything in @cmdoutput is because it can't find the '#'. I have been playing with timeouts for a while. Here are my logs:

$ cat /tmp/input.log 
Trying to connect to tftp server......
TFTP put operation failed:No route to host                            
+              
Trying to connect to tftp server......
TFTP put operation was successful                                     
+              
Copy complete, now saving to disk (please wait)...
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$ cat /tmp/output.log 
copy startup-config tftp://192.168.0.206/switches/conf/172.16.2.11-con
+fg vrf default


copy startup-config tftp://192.168.0.206/switches/conf/172.16.2.11-con
+fg vrf management
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I added a few '\n' as you can see above with the same result. when I run the commands manually on the switch I get the following(it takes about 1-2seconds to run manually):

ewfmb_179_cc7_b# copy startup-config tftp://192.168.0.206/switches/con
+f/172.16.2.11-confg vrf management
Trying to connect to tftp server......
Connection to Server Established.
TFTP put operation was successful                                     
+              
Copy complete, now saving to disk (please wait)...
ewfmb_179_cc7_b#
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I send the command twice only because some switches use a management vrf and others do not, that is why you see the first output error with 'no route to host'.