Adjust vivo MTK 6771 cpu boots
with this 3.21 update,all the vivo MTK 6771 working with preloader driver
mean with this you can unlock by power off mode,in the software select X21i or Y97 z3i...
Disconnect Mobile...
re-input Mobile Battery...
then connect usb to computer....
Make Sure your Mobie have power...
>>> Searching Mobile Com Port...
Get Mobile Port: COM10
>>> Driver Identifying name:
MediaTek PreLoader USB VCOM_V1632 (Android)
>>> 手机连接正常...
CPU_NAME: MT6765
>>> Reading Mobile Hardware info...Success
HW_Inf: 8A00 SW_Inf: CA00
>>> Initializing Mobile Phone...Success
>>> Downloading boot1 to Mobile...Success
手机状态:SYNC...
手机状态切换为 :brom... 手机状态为: preloader
确认手机状态为 :brom...
Initializationing BOOT2 ...
=====================Flash 类型(eMMC)==============
Flash Size: 0x0000000E8F800000 -> 59640M
Flash ID: 0x150100475036424D
Flash Name: Samsung_Unknow
Flash Code: GP6BMB
>>> switch hight speed re,wr port...
Get Com Port is: COM13
>>> Driver name:
MediaTek DA USB VCOM (Android)
>>> Reading Mobile GPT data...Success
>>>>Write Data to Mobile userdata ...
all is done!!! 
Thanks MRT KEY TEAM