You do not mention Greame the size of the lathe you have, but on my website there is a very detailed explanation of how to machine both Stuart H10 and V10 engines using just a lathe plus a drilling machine, and even gives a suggestion how to manage without a drilling machine.
The lathe being used is a Hobbymat MD65 having a 65mm centre height. It does not have a T slotted cross slide or a vertical slide available for milling. The pages can be found here
If you wish to use a vertical slide then you should go for a toolmakers style vice. The reason for this is that you can uses the vice's mounting method to move the workpiece into place for machining the part, leaving the full vertical slide traverse for the machining operation.
The vices offered for use on vertical slides are in my estimation ridiculous as they only hold very small parts and can only be mounted in a limited number of fixed positions. See my pages here In particular, see photographs 4 and 5 on the third page which shows the situation even exists on the long standing vertical slides for the Myford. It is about time, in my estimation, that some common sense was applied to the situation for the owners of vertical slides and very small milling machines. If one has purchased a very small lathe then you do not want the accessories to limit its capacity ever further.
Harold