Approach - getting thereLong version
Argentina - Mendoza to Potrerillos. Good bus connections between Mendoza and Potrerillos.
Hitch or go with a pre-booked vehicle to Vallecitos ski station.
Walk for 1½-2h on good paths to the meadows of Las Vegas where there are really good camping alternatives. Fresh water around.
On the right hand side of the valley you'll see San Bernardo.
First part of the route
Start from Las Vegas. Look at the peak to locate a switch-backing path up a sandy area which leads to the fore-peak’s left shoulder. It should be clearly visible if not snowy. Walk towards the path and the grassy low ridge, which marks the edge of the Las Vegas meadows. Have a good look at the river below. Some places are good for crossing it, some are close to impossible if the water levels are high. The slope leading down to the river can be extremely slippery if it has rained.
When on the other side of the river, look for the path. In the beginning of the walk you can go anywhere, but it’s of great help to stay on the path as the surrounding sand-fields are very taxing to walk on. When on top of the switch back section continue to you right towards the fore peak. It gets more rocky after some time and the path can be hard to find.
At the rocky section, you have three choices:
1) You find the path and ascend on the semi-technical rock. I saw some people roping up here and most people left their walking poles behind.
2) You find you own way up the rocky section, which can be tricky, but if you’re a rock climber, you will find it interesting as there are some nice, not-too-hard sections which can be negotiated without ropes.
3) You walk on the scree below the rocks. It is possible, but the slope is quite steep and you’re better off going straight up in the rocks.
The whole route, seen from below
You’re now below the fore-summit or more correctly; a lower peak on the San Bernardo ridge. Move to your left, below the face, in the scree all the time traversing slightly upwards and if possible along the path which can be hard to find in the rocky terrain. When the slope gets slightly less steep, move up to the saddle in between San Bernardo and the fore-peak. On the saddle you for the first time have clear views of your target. Move towards the highest point of the peak. It may look steep and impossible to climb it without gear, but there are some paths, not very steep at all. The one to the right is preferable as it is the least exposed. Move carefully up a gully until you have past the gravel and rocks and are on solid rock. The right hand side offers the most stabile rock and it is also the least steep. After four to five easy moves, it flattens out and you have an easy walk to the summit. The summit is marked with a large cross, in memory of two climbers who died attempting a difficult route on San Bernardo.
The normal route is mostly a scramble, but the use of walking poles is an advantage to at least the first rocky part. Apart from that, the normal wind and water proof clothing, sun-glasses, plenty of drinking water is a must. The weather conditions change really fast here and you may want to bring a GPS to find you way back to Las Vegas if the visibility drops quickly.
A Book With More Route Information
Mountains of light/Cord of the Silver, by Alejandro Geras have information about other routes on the peak.