Abstract Factory + Strategy pattern for parsing log files in multiple formats — improvement suggestions?
Few thoughts about your code:


your case resembles more Abstract Factory pattern more than Strategy pattern. ILogFactory is your Abstract Factory while SmithsLogFactory, VividFactory etc are yours Concrete Factories. ISessionLog, ITipLog and IStatisticsLog are Abstract Products and, for example, TipLog with L3TipStrategy inside is your Product.
Because I see this case as Abstract Factory, I would add derived versions of ISessionLog, ITipLog and IStatisticsLog for each manufacturer. Constructor of each of those classes should take care of details - like creating proper version of Parser subclass. 
When using Abstract Factory I always like to provide * construction parameters* to the methods, which can be used to control concrete products creation - and here is the place where I would handle issue with different versions of the logs from the same manufacturer.


Draft summarizing my thoughts:

    public class LogFactoryParameters
    {
       // ... for example, Version
    }

    public class L3LogFactory : ILogFactory
    {
        public ISessionLog CreateSessionLog(LogFactoryParameters parameters)
        {
            return new L3SessionLog(parameters);
        }

        public ITipLog CreateTipLog(LogFactoryParameters parameters)
        {
           return new L3TipLog(parameters);
        }

        public IStatisticsLog CreateStatisticsLog(LogFactoryParameters parameters)
        {
           return new L3StatisticsLog(parameters);
        }
    }