No, 3.74 does not change the core ability to use plugins (taiHENkaku). Most plugins designed for 3.65-3.73 work on 3.74. What is the advantage of 3.74 over 3.65?
Rename the downloaded file to PSP2UPDAT.PUP if it isn't already. ps vita 374 firmware link
The PS Vita 3.74 firmware update includes several key features and improvements: ps vita 374 firmware link
Alternative 3.74 Download MD5 Hash: f2c7b12fe85496ec88a0391b514d6e3b How to Install PS Vita 3.74 Firmware (Two Methods) Method 1: Official System Update (Easiest) ps vita 374 firmware link
The absolute safest method is downloading the update file ( PSP2UPDAT.PUP ) directly from Sony's official PlayStation support infrastructure.
Transferring content directly between a PS3 and a PS Vita via USB is no longer supported due to these security changes. For Homebrew and Hacking