Please accept my apology for the delay in response due to workload. For the Vref, you may refer to Cyclone 10 GX datasheet on page 16 under section "Table 13. Single-Ended SSTL, HSTL, and HSUL I/O Reference Voltage Specifications for Intel Cyclone 10 GX Devices" --> https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/cyclone-10/c10gx-51002.p...
I attached the screenshot as well for your reference. 😊