In Coins Configuration I created Vertcoin with two different pools (both alive). Then I selected vertminer for Scrypt-N mining and placed the vertminer-0.5.2 folder (renamed to vertminer) in the folder appdata/local/multiminer/miners. But when starting mining vertcoin, MultiMiner always choses BFGminer as miner.
Then I did the same thing with SGminer. But again, MultiMiner uses BFGminer.
What should I do to force MultiMiner to mine Scrypt-N with SGminer?
You don't need to download and place any miners anywhere. MultiMiner does that for you.
Add a Coin configuration for Vertcoin. Make sure that it is set to Scrypt-N. Choose Vertcoin for your devices (using right-click) and click Start to start mining.
If MultiMiner is using BFGMiner, then it sounds like you didn't specify Scrypt-N as the algorithm when adding Vertcoin. You could try removing and re-adding the Coin configuration. Or use CoinWarz instead of CoinChoose, since CoinWarz already has Vertcoin with the correct algorithm.
I did it the same way icy lettuce did it and it works great for me. He may have forgotten to Save and Restart at the right points and defaulted to sgminer. I had a VertMiner.conf but Mum 3.01 said that was a no-no and renamed it. I just converted to a command line format and defined my pools in Add Coin. It switches and mines great.