A certain value for vref will give a certain current (amps) value for the motor output. The exact math relation for the drivers you have is something to research yourself, e.g. if you use pololu drivers try pololu site.
The 0.4A value points to a high resistance (/high impedance) motor type (12v?). Cant be sure because the other information on the motor label is missing. Also the actual vref setting is missing, so speculation is in order.
As everything appears to work, check if you dont have missing steps, e.g. move to a point and mark it down, move away and make 10+ other moves in both directions and at speeds, then return at the point and see if there is any difference, if it is then perhaps its missing steps. Hard to actually see, and its better done with a comparator tool, but probably can be done with other means. If it doesnt miss steps leave it like it is. If it does miss steps, assuming vref is set for 0.4a you can perhaps set it up to like ti give 0.57a current peak and try again (perhaps datasheet might mention that at ustep the current is 70% of the full step value). You could try pusing the current envelope a little further because the 0.4a is dc or average, and driver sets the peak and peak is not average, sort of at microstep many of the positions will have actual lower peaks. Also perhaps the lowest of those values will be too low. However is not much room to push current up. If resistance is lets say 30 ohms, then it doesnt matter if you set vref to give 1a, since with 12v/30ohms, it can not actually grow to 1a -ever. You would need 30v psu to have a 30ohms to reach 1a. Also if the resistance~impedance is too much then perhaps the current just wont have enough time to grow sort of speaking at high speeds (rpm) the motor may loose torque so might need to set the max speeds and acceleration values to lower than usual.
The 0.4A value points to a high resistance (/high impedance) motor type (12v?). Cant be sure because the other information on the motor label is missing. Also the actual vref setting is missing, so speculation is in order.
As everything appears to work, check if you dont have missing steps, e.g. move to a point and mark it down, move away and make 10+ other moves in both directions and at speeds, then return at the point and see if there is any difference, if it is then perhaps its missing steps. Hard to actually see, and its better done with a comparator tool, but probably can be done with other means. If it doesnt miss steps leave it like it is. If it does miss steps, assuming vref is set for 0.4a you can perhaps set it up to like ti give 0.57a current peak and try again (perhaps datasheet might mention that at ustep the current is 70% of the full step value). You could try pusing the current envelope a little further because the 0.4a is dc or average, and driver sets the peak and peak is not average, sort of at microstep many of the positions will have actual lower peaks. Also perhaps the lowest of those values will be too low. However is not much room to push current up. If resistance is lets say 30 ohms, then it doesnt matter if you set vref to give 1a, since with 12v/30ohms, it can not actually grow to 1a -ever. You would need 30v psu to have a 30ohms to reach 1a. Also if the resistance~impedance is too much then perhaps the current just wont have enough time to grow sort of speaking at high speeds (rpm) the motor may loose torque so might need to set the max speeds and acceleration values to lower than usual.