#Methods
###1. List comprehension
As mentioned by Pycoder in the comments below, using operators is often slow. So a quicker and simpler (and X2 faster) method to get all the verts that belong to a certain vertex group index:
vg_idx = 0
o = bpy.context.object
vs = [ v for v in o.data.vertices if vg_idx in [ vg.group for vg in v.groups ] ]
###2. bpy.ops
You can also use the bpy.ops.object.vertex_group_select operator in edit mode:
import bpy
o = bpy.context.object
bpy.ops.object.mode_set( mode = 'EDIT' )
# Set the first vertex group as active:
o.vertex_groups.active = o.vertex_groups[0]
# Deselect all verts and select only current VG
bpy.ops.mesh.select_all( action = 'DESELECT' )
bpy.ops.object.vertex_group_select()
# Now the selected vertices are the ones that belong to this VG
vgVerts = [ v for v in o.data.vertices if v.select ]
###3. Optimization if only one group
If there is only one group, this is a more optimized way to get the verts that belong to it. Note that this only works if there is only one vertex group.
vertices = [v for v in obj.data.vertices if v.groups]
#Performance testing
###1. List comprehension
With the default cube (8 vertices), this was timed 1,000,000 times, and had a minimum value of:
0.000003528 seconds
With the default cube subdivided 8 times (393,218 vertices), this was timed 1,000 times, and had a minimum value of:
0.1528 seconds
###2. bpy.ops
With the default cube (8 vertices), this was timed 10,000 times, and had a minimum value of:
0.0003289 seconds
With the default cube subdivided 8 times (393,218 vertices), this was timed 100 times, and had a minimum value of:
0.8849 seconds
###3. Optimization if only one group
With the default cube (8 vertices), this was timed 1,000,000 times, and had a minimum value of:
0.00000222 seconds
With the default cube subdivided 8 times (393,218 vertices), this was timed 1,000 times, and had a minimum value of:
0.0659 seconds
Here is a graphical (but less accurate) model if the performance. Each point is the min of 100 iterations.
Another representation of it:
Because the higher subdivision values make the lower end almost meaningless, we can use a logarithmic scale to see everything a bit more clearly:
#Additional links
Checking if a vertex belongs to a vertex group in python
How can i get the weight for all vertices in a vertex group?
How to assign vertex groups and bone weights through Blender's Python API?
How can I get the names of the vertex groups these vertices are in?