Function write_discriminator 

pub fn write_discriminator(
    data: &mut [u8],
    discriminator: u8,
) -> Result<(), ProgramError>

Write a discriminator byte to data[0].

Call this after allocating account space via a system program CPI and before writing any other fields. Pairs naturally with DataWriter for the rest of the layout:

let mut raw = new_account.try_borrow_mut()?;
write_discriminator(&mut raw, VAULT_DISC)?;
let mut w = DataWriter::new(&mut raw[1..]);
w.write_u64(0)?;
w.write_address(&authority)?;