1. Open a blank spreadsheet
2. In the Sidebar, click on the Styles icon to open the Styles pane.
3. The pane initially shows Cell Styles (first icon on left at the top).
   
4. Right click Default, select Edit style
5. In the dialogue that opens, select the Fonts tab. Select your desired font and font size, change nothing else. OK
6. Click File > Templates > Save as template
7. In the dialogue, type a suitable name, select an appropriate location such as My Templates, tick the box Set as default template. Save

To remove the default template status if no longer needed, click File > Templates > Manage templates. FInd the template with a green tick, right click it and select *Reset default. Or right click another template and select Set as Default to make that default template.

Source: How can I change the default font in Calc

A novice user guide on VLOOKUP function in LibreOffice Calc spreadsheet program with examples and things you need to know.

Source: VLOOKUP with Examples in LibreOffice Calc – libreofficehelp.com