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.