Merging multiple vector layers to one layer using QGIS

You can use MMQGIS tools for merging...

The merge layers tool merges features from multiple layers into a single shapefile and adds the merged shapefile to the project. One or more layers are selected from the "Select Source Layers" dialog list box and an output shapefile name is specified in the "Output Shapefile" dialog field.

Merged layers must all be the same geometry type (point, polygon, etc.). If the source layers have different attribute fields (distinguished by name and type), the merged file will contain a set of all different fields from the source layers with NULL values inserted when a source layer does not have a specific output field.

i hope it helps you...

You can use the 'merge shapefiles to one' function under the menu vector|Data management tool. You can merge all files in the input directory or select specific files in the input directory.

The same applies as for MMqgis tool: merged layers must all be of the same geometry type. Also, if the source layers have different attributes fields, the merged file will contain all fields, but with NULL values inserted when a source layer does not have a specific field.

You can also use the v.patch module under GRASS GIS commands. It's available in the processing toolbox.

You can search for it when the dropdown at the bottom of the Processing Toolbox window is switched from "Simplified interface" to "Advanced interface".