You can take the valley train between Chamonix and Servoz, particularly useful if you want to avoid the 300m height gain for the cycle back up, or there’s a few ways to cycle between them.
If you want to stay on the tarmac, follow the road through Les Houches and past the Prarion gondola. Keep following the signs for Vaudagne and then Servoz until you reach the village.
If you’d prefer to head off-road, at the Prarion gondola in Les Houches take the 1st exit from the roundabout and ride downhill past the ski area car parks. Keep going down the road, past the tennis courts until it becomes a 4x4 track. Now just keep riding along this track near to the railway line until you reach the Vaudagne train station, which you’ll notice is not particularly near Vaudagne! Fortunately the rest of the route along a nice piece of singletrack is signposted all the way to the road, where you turn right and ride down the tarmac, and under the motorway. For the last link into Servoz, head for the hotel La Fontaine then take the trail signposted Servoz just before it’s car park.
Once in Servoz there are a huge number of trails to try. If you are looking for something simple then keep going through the village on the D13 road. The road falls and rises for several km and you might want to just keep going on the road and explore the small villages along the hillside between the Fiz cliffs and Le Fayet below. If the call of dirt is too strong then about 1.5km outside Servoz you’ll cross a small bridge (the Pont du Nant Noir) with a gravel car park immediately after it and, if you have very good eyesight, a small trail signpost. From here follow the 4x4 track at the back of the car park down the hill to Chedde, the town at the base of the hill. You get some amazing views of the valley from here, from a direction that you don’t normally get either.
Once back on the tarmac through Chedde you can get the train back up from the station or continue along to the main station at Le Fayet. Alternatively you can take the Tramway du Mont Blanc and ride some trails above Les Houches.
For something more challenging, both up and downhill, cycle through Servoz to the Gorges de Diosaz then up the hill to Le Mont. The road is well surfaced, but it’s still a fair climb. Fortunately once you reach Le Mont there’s a water fountain in the middle of the road to help you cool off and prepare you for the final ascent along the road towards Lac Vert. Fortunately you don’t need to cycle all the way up to Lac Vert (though you can if you wish) but instead as the tarmac road turns to dirt trail and you reach a car park, take the trail on the left signposted "Vieux Servoz par Sentier 0h35" and, at the very 1st corner, turn hard left onto singletrack.
From here it’s fast flowing singletrack all the way down to the D13 road where you can either turn left to ride back to Servoz, or right to follow the above track to Chedde & Le Fayet. There’s a couple of junctions on the track, turning right at every one might be the best, but the only way to be sure would be to cycle back up again and try the others, just to make sure…