Assuming your Vector3 type is a struct, and all of its members are blittable (which singles/floats are), you should be able to run them through the Marshal.

I’ve not tested it, but something along the lines of this should work:

private unsafe void Writes()
{
byte[] buffer = new byte[sizeof(Vector3)];
fixed (byte* b = buffer)
Marshal.StructureToPtr(new Vector3{X = 1, Y = 1, Z = 1}, new IntPtr((int)b), false);
byte[] buff = new byte[sizeof(Vector3)];
fixed(byte* b = buff)
Unsafe.Write(b, new Vector3{X = 1, Y = 1, Z = 1});
}
struct Vector3
{
public float X;
public float Y;
public float Z;
}

Where the Unsafe variant is much, much faster.