First, make sure that the repository containing the NMaven assembliies and vsinstaller is located on your local file system. Typically, you would only do this for testing or for installing from a non-standard repo location. Next, configure your settings.xml file to point to your repo.
<settings>
<profiles>
<profile>
<id>nmaven.snapshots</id>
<repositories>
<repository>
<id>local</id>
<name>Maven Plugin Snapshots</name>
<url>file://C:\m2\repo</url>
<releases>
<enabled>true</enabled>
</releases>
<snapshots>
<enabled>true</enabled>
</snapshots>
</repository>
</repositories>
<pluginRepositories>
<pluginRepository>
<id>local</id>
<name>Maven Plugin Snapshots</name>
<url>file://C:\m2\repo</url>
<releases>
<enabled>false</enabled>
</releases>
<snapshots>
<enabled>true</enabled>
</snapshots>
</pluginRepository>
</pluginRepositories>
</profile>
</profiles>
</settings>
Due to a bug in Maven, if you are remotely resolving a plugin that doesn't exist in central, you will need to install from a directory containing a pom.xml file. So if you have a pom.xml handy, goto that directory from the command line. If not, you will need to download one and goto to the download directory from the command line. Now type the following from the command line:
mvn org.apache.maven.dotnet.plugins:maven-vsinstaller-plugin:install