There are three modules you can use to convert a raster to a vector map, depending on the type of objects you wish to convert.
If you want to convert raster lines to vector lines, you first have to make sure that the raster lines are thin enough to be transformed into only one vector line respectively, and not into several parallel lines. "Thin enough" actually means having a width of only one cell. The module to use for this operation is r.thin.
Once you have thinned the raster lines, you can use r.line to transform the raster data into vector data. Don't forget to run v.support after r.line. The quality of the resulting vector lines depends on the resolution of the original raster data.
For transformation of raster areas into vector area lines you can use r.poly. If you use the "-l" option, the module will smooth the corners to avoid the blocky appearance of raster areas. Be aware that, depending on the resolution of the raster map and of the original data, this smoothing might induce a certain error. Again, don't forget to run v.support after the transformation. The cell category values for the raster map layer will be used to create attribute information for the resultant vector areas.
You can transform a raster surface into vector isolines (or contours) with the r.contour module. If you just give it the interval between lines ("step" command line option), it automatically determines the other values. You can also define a fixed series of levels at which you would like to see isolines ("levels" option). The module automatically runs v.support for you, so you don't have to worry about that.