Silent alarm is possible to set only in per-alarm setting for each individual alarm. You can imagine the consequences of this option enabled by mistake, especially when backup alarm was silent as well…
If you are using vibrations only, you can set a delay for sound start, which can be set as a default for all alarms. The maximum delay is 24 minutes, so it should be enough for being waken up by vibrations before the sound even start.
You can combine this with gentle volume increase set to maximum (24 minutes), which will give you 48 minutes of vibrations before the alarm volume reaches the max.
Settings → Alarms → Alarm sound start (depending on your settings, might be hidden under Advanced menu, just right bellow Ringtone).